Not Equifax per se, but my understanding from dealing with background checking companies is that this pattern is to discourage people from sending sensitive information over email. 99% of people won't have a clue about how to encrypt email, nor will they think twice about sending an unencrypted attachment. Whereas a sanely coded web portal can enforce this.
The irony here is that its done for "security". But if the app itself has an unpatched vulnerability...
Ps I once did try sending an encrypted PDF to such a company, and instructed them to phone me for the password. This was apparently way over the heads of the person assigned to my case.