The three philosophers you cite are among those commonly cited as good writers! I cited Nietzsche myself above. I suppose it depends on how you define 'good' writing. In the case of literary style I think for most people it boils down to there being some amount of creativity and play on the level of language itself (i.e. see Nietzsche) which is disconnected from the content. e.g. I'm sure Bertrand Russell was capable of comprehending Nietzsche's ideas(though the actual Russell never would have) and could just as easily have written a treatise on eternal recurrence or the history of morality as Nietzsche conceived of it. Point is, Russell's rendering of these ideas would have been entirely different, even though they are employing the same medium of communication. Another person could very well have formulated the same ideas as Nietzsche, but probably wouldn't have conveyed them with as much creativity and grace. There's writing that shines as writing and there's writing that is in service to some goal, i.e. the clear work of modern analytic philosophers--they'd hardly gain any attention for literary ingenuity or clever turns of phrase, but you are correct that if our judgement is based solely on the effectiveness of the communication of the idea, they are in fact quite fine writers. And actually, in this sense Nietzsche would be pretty poor, because as Analemma mentioned his loose play with metaphor, while incredibly delicious on the level of style, does not communicate very well, if the idea is that a 'good' piece of communication delivers a single idea all the consumers of that communication can agree upon.

So perhaps I'd revise the claim to something like, most philosophers are not good stylists (that is, they don't frequently engage in play at the level of language, as a skilled poet or essayist might)

As to your second point yes. There is always a reason. You can question that reason--i.e. you may think the reason is simply that academics were bored and decided to laud the first sap whose writing they came across that day--but this is a pretty absurd claim. You'd essentially be stating that a whole domain of tradition, practice, and procedure which organically grows and evolves, and I might add, in almost logical progressions at times, was ousted by the whims of one foppish professor who gamed everyone into liking something he liked simply because he liked it and was impassioned enough about it. When you plunge into a field of art, technique is often the criterion and leveling factor. For instance, you say you have never heard a convincing argument as to why Pynchon would be considered worthy of canon status--well, I'm not going to be so absurd as to claim he'll still be there in 200 years, but if you have knowledge of literary craft the reasons why he's there now are pretty clear--his maximalism is both well crafted and unique and his style is a turn away from the still dominant style of american literature (Hemmingway based minimalism) which is positively refreshing (the same could be said for DFW, who was consciously, I believe, rejecting the minimalistc style--I recall he wanted to move away from his maximalism too around the time of his unfortunate death). It's because he utilizes traditional structures and devices in a unique way--but in a way that is importantly still comprehensible under the lens of this tradition. Take for example the dawn of the unreliable narrator--it utilized a familiar technique in the field of literature, namely the narrator, and modified it in such a way as to generate interest--as to who gets the credit for such developments--well, it probably comes down to luck and knowing the right people. Yes, all these aesthetic considerations are ultimately conventional and wispy--as all human values tend to be--but they nonetheless obtain, and traditions develop, evolve, die, or persist. There are indeed plenty of 'rules' when it comes to art forms--that is how, at the most basic level, for instance, I know that something is a painting and not a piece of music--the medium and form follow particular restrictions (and then we have great fun blending and challenging these notions).

That's why any critic worth his salt often delves into art history, the artists personal development over a series of works, and analysis of form and technique over simple and baseless value judgement. I may wretch at every Jackson Pollock piece I come across, but if I am educated in the discipline of painting, its history, and its techniques, I can understand where his pieces fit into the narrative of painting history, what they challenge, what they change, and ultimately how unique his forms are and what they communicate within this context. If I dislike it, if I find it shouldn't be considered art--well I have to argue it from this perspective, from within the game of homo sapiens art history. This is why anyone who makes a snap judgement against such artistic efforts and says things like "anyone could do that, it's not art" always comes off sounding dumb and uncultured--they are treating the work entirely out of context and clearly lack an appreciation for the medium as a whole--unless of course they provide reasons which leverage knowledge of this medium.

At root our aesthetic explanations and investigations ultimately boil down to our base value judgements of simply "I like this thing or don't"--but artistic forms exist because there are elements of these traditions a large number of people can generally agree they appreciate, can describe with a common language, and can critique in comparative ways.

Thus the cannon isn't a good judge of anything other than what your precursors believed should be appreciated. It's essentially the historical development of a shared value judgement, or a shared human prejudice. So of course you can repudiate the whole thing. But at that point you are no longer even engaging in that art form--or at best you are engaging with blinders on, and any aesthetic mastery you manage to pull off is largely lucky and unconscious. You are starting from a different base. you are playing your own game. Thus you shouldn't be too upset when other people don't appreciate what you do, or your work isn't considered interesting. You're not even speaking their language.

It's funny to compare different advice on reading material in this context. Faulkner suggested you ought to read everything. Schopenhauer suggested bad books ought to be avoided like poison--the old garbage in garbage out principle. Both work, but if you forget your contexts and say, suggest to a film critic that the marvel movies rival Citzen Kane, they won't even begin to agree unless you layout a sophisticated argument that appeals to the criterion generally recognized by adherents to the art form, elements of cinematography, the quality of the script, etc. etc...

Wittgenstein's notion of language games, I think, is very informative when applied to the realm of aesthetics.

Sorry for the lengthy reply. You got me on a role. Good stuff.