The details have been left vague as to why.
MultiVersus

A day ago, we were informed that the Season 1 Battle Pass for MultiVersus would cost you 950 Gleamium for 50 tiers of unlocks. Today, we're now learning that the first season for the popular platform fighter is actually getting delayed.

The official MultiVersus Twitter account informed everyone that they are delaying the start of Season 1 along with the release of Morty "to a later date." A reason for the delay was not provided. Even a vague future window about when it might be released was also not provided.

It's not clear either whether or not this has any impact on the "open beta" status of the game as it was planned to leave beta when Season 1 started on August 9. Either way, the game is and will continue to be free-to-play and progress carries over, so it's not like it matters in the slightest whether it's considered beta or not.