Because they asked nicely for the documents that Trump had no right to retain and were denied.
Because they then issued a subpoena for the documents that Trump had no right to retain and it was ignored.
So then they raided his home for the documents. Because all other means of obtaining them had failed, and because Trump, former president or not, had no right to have them.
And nothing Hillary did or Obama did or anyone else did, or your flagrant misunderstanding of what they did, gave him that right.
So either you're asserting that Trump
didn't have the documents in question, or, by merit of being a
former president (yes, former, as he's not the "rightful president" regardless of his claims, which I'm sure is a whole other can of worms from Trump you're ignoring,) should have been able to retain them for however long he wanted in any way he saw fit, and he should have expected no recourse for having done so.
Those are your only two options here.