The trend for Flutter is a straight line going up and React Native is going down in popularity.
The trend was obvious many years ago, and just recently Flutter is now surpassing RN in nearly every poll.
The simple answer is that when people have a problem or question about Flutter they simply key in Flutter. With RN they can ask about React, JavaScript, typescript, RN itself or one of the many components you use to make an app as it's the sum of a lot more parts.
So if Flutter didn't get more searches here I would be surprised and concerned.
The longer answer I'll just tease but suffice to say it's more complex than above.
It can come down to lots of things including the competency/experience of developers and maturity. As an experienced developer I might need to search for or ask about what I'm doing maybe 2 times a week. For someone getting started that might be that many times an hour. If you just looked at the two of us as a blob it might seem that whatever the other is using is more popular despite it being 50/50.
So yeah, number of searches or stack overflow questions or Reddit posts or whatever it is does not equal popularity.
I imagine I am having a harder time swallowing this logic than you are looking at raw numbers.
What you said was an example of one factor as to why it's more complex than the simple form for people who really want to dig down deep but on its own it's not an answer