Why do AT&T, Verizon support D Block reallocation?

One reason (among others) is that service could sometimes be degraded for their customers if the D Block is auctioned off to commercial providers.

Situations could arise where their customers would be unable to make calls. That's unacceptable to the companies. The AT&T press office emailed that it would be "unfair" to its customers. Proponents of this plan, however, say service would not suffer greatly. Instead, both carriers have backed a proposal reallocating the valuable D Block of spectrum directly to public safety, so the network-sharing issues don't arise in the first place, although this option is more expensive for the government. That argument is winning.