Well, only a couple of reasons that I can think of.
1. DIVX (not DivX;)) made a bad name for itself trying to be a cash cow for the big production studios with the "buy the disc and rent it over-and-over again" concept that was founded by the CEO of Circuit City. So, they might find that to be bad marketing for the time being.
2. I was under the impression that the "standard" for DivX is still a bit flexible, so building a unit cheaply that would either read more than one standard/sub-standard or have upgradeable firmware options would not be feasible at this time. With WMA, MS can dictate the lifecycle and guarantee "X" amount of lifespan for the codec.
Any other opinions?