Why do you think it's rare? Unless it's a personal project not meant for others, why would they not want more people to use it and get more popular?
To me this is like people getting upset when their favorite indie band "sells out", even though their increased popularity is always seen as a much bigger positive than any potential downsides of the "sellout".
My impression is that people are motivated by solving problems and filling gaps in the existing landscape. Popularity is not a goal, usually. Why would it be?
More contributors, community building, more competition/innovation, I could think of many reasons... but not wanting something to become more popular is quite perplexing to me.
That’s rare, and is a very difficult motivation to understand. Such projects are also usually a mess.