the issue comes down to stakes.
If you introduce a multiversal issue, you immediately neuter it's importance. "so what if things go terrible in this timeline. There are others

' becomes the immediate response, even if it's subconscious.
So, either it becomes a 'save all the universes' in which case the scope gets too big, or it's a big-ass Ye Shrug when stuff starts getting tough because all the important characters can just relocate to a less-bad timeline, since we know they exist now.
It's just better for the telling of a story to keep it grounded in, at least, a singular reality.
this in terms of telling a story with conflict. If it's more of a plot device in the background of a slice-of-life, romance or just an adventure it can be fine. But if you're trying to give it gravitas, quickest way to
not do that is to have a multiverse. It's a nihilism dream being told that the whole universe as you know it is 1 of many and yours doesn't matter in the grand scheme of things