You can’t use Direct File if:
If your wages are more than $200,000 ($168,600 if you had more than one employer).
You file as Married Filing Jointly, and your spouse’s wages are more than $200,000 ($168,600 if your spouse had more than one employer).
You file as Married Filing Jointly, and you and your spouse's wages are more than $250,000.
You file as Married Filing Separately, and your wages are more than $125,000.
$168.6k is suspiciously the same as the (2024) social security cap, so they probably didn't handle that edge case.
$250k is the start of NIIT, so they probably didn't handle that case either.
My guess: since the tax code is so complicated, they probably wanted to support the simplest and most common filing cases in the first release. Handling all the edge cases would delay the launch, and prevent them from collecting feedback. If the feedback and demand was positive for v1, they could expand the surface area in the future.
Unfortunately, it didn't last long enough
[0] https://www.census.gov/data/tables/2024/demo/income-poverty/...