CTR_DRBG versus OFB_DRBG
No OFB_DRBG isn't directly vulnerable to attacks.

However from a theoretical standpoint there is one point speaking against it and in favor of CTR_DRBG.

Speed. CTR can be parallelized (can encrypt counters in parallel), OFB can't (encrypting state over and over again)

So the main reason is speed I'd guess and of course adding OFB_DRBG wouldn't have added anything to the portfolio, as CTR does the same job, but better.