There's a reasonable argument to be made that they were worried if they allowed Texas to sue, then it would open up any other state to sue other states about whatever they wanted (eg someone on another thread suggested in future, Texas maybe gets sued by blue states for "unconstitutionally banning abortions" or whatever).

We don't really have a way to know right now if it really was something like that, and they are just waiting for a case they consider "better" to come along - or if they genuinely just want to sit it out and don't believe that they should get involved.