For consumer motherboard OEMs, only AMD effectively has ECC support (Intel's has been so spotty and haphazard from product to product), and of AMD users, only a small number care about ECC.

So motherboard companies, being resource and time-starved as they are, don't make it a priority to address such a small user-base.

If Intel started shipping ECC on everything, it would go a long way towards shifting the market.