I don't think it's one thing, but there was definitely a sense that Onward (which did little in the brief time it was in theaters) and then the D+ films were all not up to Pixar's standards. Not that they were bad per se but that they were below the high standards previously set by Pixar. Also, Turning Red in particular seems to be somewhat divisive. So, I thing over that time, there was a shift in people's attitudes of "Pixar is so good, I'm going to run out and see all their films" to "well, I guess I'll see that Pixar film when I get the chance" then Lightyear was definitely not well received (and Strange World on top of that which contributed to diminishing the overall Disney animated brand, which Pixar is a part of) and I definitely think there is a trend where Pixar is now amongst the pack of just "regular" animated films and not its own special draw.
And, yes, the pandemic (which has dampened box office sales) and streaming (which has led to more people "waiting" for films) play a role too but Pixar's drop was more precarious than others. Of course, when you have a higher ledge, it's a bigger fall so there's that.
But I do hope that Elemental's generally good word of mouth and strong "legs" will help to rehab Pixar's image. There's always going to be a lag where you have to have something change the attitude and it's the next film that will get the "benefit of the doubt" and potentially a better opening/strong pre-sales and anticipation.