The number of slots doesn't matter but the number of channels, i.e. x86 board have 4 slots but only 2 channels (through nomenclature is a bit of a mess as as depending on context channel might mean something else).

More memory bandwidth increases the price/size of the IO module on the CPU but I don't think that's a major reason.

One of the major reason AFIK is to keep the number of channels low is that it directly maps to the number of pins a socket need to has and the number of high quality lanes which need to be routed independently equally long and chided from inference from e.g. power lines to the CPU.

So especially e.g. for server design chip vendors balance the benefit of more memory channels with the cost of it increasing socket size/pins, complexity of mother boards etc.

And while on the paper doubling the memory channels doubles the memory bandwidth it doesn't at all means you RAM is twice as fast. It's similar to the difference between duplicating the number of cores vs. duplication the per-core IPC. While all applications profit from more bus width the degree might differ hugely. Oversimplified super highly parallelizeable tasks (e.g. much of GPU tasks and many AI tasks) tend to also be better at taking advantage of more bandwidth through more channels but many other taks less so. This means that the degree to which more channels will yield benefits highly depend on the task the CPU vendor expect is majorly run on it. And the tasks which most benefit from it already tend to be run on GPUs anyway.

Apples M1,M2 CPUs one the other hand have a much much smaller cost problem wrt. more memory channels due to how they stack ram on top of the CPU (I think) and they happen to also have much more memory bandwidth by having more bus width.

But stacking RAM on top of CPUs has it's own issues. Like for example if the RAM dies the whole CPU is dead and you not being able to upgrade the RAM. Doesn't matter that much for apple, but matters a lot for AMD/Intel which core focus is the server center, not desktop CPUs. Having to replace a 10k CPU and multiple TiB of RAM because a single ram segment fails just isn't acceptable. Additionally for some use cases instead of stacking RAM on the CPU you get as much or more benefit by stacking tons of L3 cache on it (AMD 3D V-cache). There also seems to be in general be more issues with thermal budges, not an issue for apple of which most devices are mostly passive cooled. Issues with manufacturing cost making it harder to provide many CPU spec+RAM amount variants (again doesn't matter for apple but does for Intel/AMD). Potential cost issues making it only suitable for "premium" products (again not an issue for Apple but for Intel/AMD). And maybe more.

So basically:

- on server (or pro-sumer,i.e. Threadripper) more channels are already used, but it's always a cost benefit balance where there could be even more channels

- on desktop, at least currently, it's not seen to be worth the price/complexity increase

Through Apples is putting a bit of pressure on the market, but outside of the high end segment it still makes little sense and it's probably not worth it to have it just for the "high end but not pro-sumer" use-case, that market it too small.

I would predict prosumer devices like Threadrippers to grow the number of memory channels with every now socket they have.

And I do predict a major technology change for "normal desktop computers" 3-6 years down the line which probably will bring more channels to desktop computers. But only then. And it's very speculative.