Because those in the Biden Administration knew they could hold on to power without triggering the 25th.
With Trump, there are a few things that are different. First, you have a hostile media that will cover a decline more closely. (Mind you this won't be the run of the mill Trump said something crazy criticism that we've heard for over eight years.) Lastly, it was well-known the Biden team did not like Harris. They might have very well been replaced with her ascendancy. Remember the Biden team thought they could have bought themselves another four years. It's possible (not saying I know) that J.D. Vance has more of a rapport with the Trump team.
No, it isn't going to be used for "mental incompetence".
You need WAY too many people on your side to pull it off.
No one cares about Trump's mental decline, the legacy media has been covering for him for years already.
I mean, sure, I guess if he starts literally running around naked on the White House lawn with a knife or something.
But section 4 of the 25th amendment is just as dead letter as impeachment.