Why would you spend money on filling a DC with older gen chips especially when you will receive the newer chips with MUCH better efficiency beats me. You either have money to burn or you will simply build a new DC for the newer chips or you simply pay so little to do this that you can afford to replace everything in 1-2 months. I can understand filling just a part of it for tests, but a whole DC...something doesn't adds up.
Just my take on it but: Because one has to start *somewhere* and get the ball rolling. The older chips were on-hand and they have working boards for them. As a bonus they get to test the DC at full load.
Newer more efficient chips/boards are in the pipeline but not on-hand so you either have a DC sitting there empty/partly used or ya plug in what you have right now and upgrade when the new designs get into full production.