A solo Joker/villain origin movie had never been done before, in just that way. It felt fresh at the time (even though it continues to age poorly and I've never once revisited it since the theater).
Batman, on the other hand, is essentially on its fourth modern iteration. I consider Burton/Schumacher one, the TDK trilogy two, and Batfleck three. Including Battinson, the final three iterations have had only three or fewer years between them. So, yeah, probably a little Bat-fatigue, and a little bit having to do with Marvel still being king as well.
And then there's definitely some lingering pandemic hesitation, but I actually actually think the situation in the Ukraine might have impacted it in the slightest as well. People have been glued to the news, there's legit talk of nukes and WWIII, gas prices are soaring, etc, and some simply might not be in the mood for a Batman movie right now, given the current state of affairs.
Also, as I've been saying for a while now, early March is just a weird time to release a blockbuster like this. In a perfect world, it absolutely should have been either an October movie (when Joker and Dune released) or a summer movie. Early March is kind of no man's land, and it's almost like, subconsciously, people aren't in-tune this time of year for a movie like this.