I think giving her a lesbian love interest would shift the entire story to focus on that plot and could overshadow any other story.
If she had been a lesbian already in the first film and it was just something matter of fact that had always been the case, then they wouldn't have to focus on it at all. She would just be a snow queen who happens to be lesbian, a bit like how Thunder is in Black Lightning.
But Elsa has been sheltered all her life and shut away from everyone else. So she's never had any kind of love interest or relationship whatsoever. That means you can't just drop that in there without giving it attention. Frozen 2 will become Elsa's coming out movie. And because of that, it will overshadow everything else and the romantic plot will become the main story.
If they want a lesbian character, it's better to do that from the start with a different character who won't need to come out but is already that way (again, like Thunder in Black Lightning).