Crooks don't want to work harder than they have to

from here

If you're looking for money then you rob banks because that's where the money is. If you're looking for personal info then you rob Equifax because increasingly that's where the personal info is.

There's actually a couple of reasons why breaching Equifax may have been easier than compiling the data

1. Equifax may not have done a good job of protecting the data (we don't know yet)
2. The more breaches there are the more work is required to collect the data from all the various sources