Two classic cases are Ian Hamilton Finlay at his own Wild Hawthorn Press and Bob Cobbing with Writers Forum. Both poets are so idiosyncratic in their approaches that no larger press would ever be able to accommodate them. Small Presses with their antipathy towards economics, marketing and all the rest of the publishing world's regular trappings are ideal vehicles to publish a work consisting of three cards in a plastic bag or a fold out sheet with one word on it. It also helps, I guess, that the poets concerned own their respective presses. Small is also containable within the head.
Conglomerate puts form and decision in the hands of the corporate accountants.
Look what happened to Chris Torrance - years of small press struggle and then, at last, grand acceptance. Paladin put him out in paperback and he was available in every bookshop UK nationwide. But that was for a mere fortnight. The company changed hands and all poetry was pulped. Famous for 15 seconds, that was Torrance's 1980 poetry fate. .