It could be either or both of two reasons: 1) Login.gov does not have identity verification. It appears like they do because most accounts are created via the Trusted Traveler programs. However, that system belongs to CBP and is not available to other agencies. ID.me has multiple different types of ID verification, including specialty affiliations like military/veterans, first responders, nurses, teachers. 2) ID.me has a larger pre-existing pool of vetted users. This means less new signups for IRS which translates to less help desk calls, etc.
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