It's nothing to do with cost in all honesty - in fact, it probably costs exactly the same.
The main reason (and basically the one that makes the choice):
-White POM/Acetal just doesn't sell as well. EKWB has tried it in the past and sales, compared to black were just not there. Blocks and reservoirs just don't get the sales in white, so it just alienates the market. Other products in white (Radiators and fans for example) do sell very well though.
Other reasons:
-It gets damaged/dirty faaaaaar quicker IMO. I actually sent a picture to VSG showing one of my white products that just hold onto dirt, black just seems far more robust (whether it is or not, can be debated all day long, but the white certainly shows up the dirt easier)
- Even if the piece was white, there is enough of the board black that it would still have a lot of black on it (all ram slots, PCI-E etc etc.). Sure, these get covered by whatever you plug into them, but once the loop is in place, the black on the block will be more subtle too.
The ONLY way I can see us doing white blocks again is if we do a full white range. Which, I would love