Why do companies stop selling reference design graphics cards?

well, basically because it sucks being in the minority, since the majority like specially designed PCB and coolers. especially the coolers, since they are almost always quieter and work better than reference design and cost only $10 or 20 more than reference. thus, as a card gets older, people stop buying the reference designs and the company choose to stop producing them. it's cheaper to produce only 1 type of product instead of two, and most company's marketing research must show that non-reference designs sell better

The people who usually by reference design cards are people who either are enthusiasts or plan to put their own aftermarket cooler on them. this consists of probably less than 1% of the people who buy GPU's and most users would like a decent aftermarket cooler on the card already.

You got point but what waterblock is compatible with those non-reference cards?

the vast majority of non-reference cards just have a non-reference cooler, the shape and holes on the PCB are all the same. the reference waterblocks will do just fine. most water-cooling enthusiasts just buy the cheapest card they can find, reference or otherwise