It simply may be overkill for some projects. No reason to get a 4 when a 3b or pico will do the job just fine at a fraction of the cost. Same deal with the 5; It’s more powerful, but it’s also more expensive and not every project needs the most recent version. The original goal of the Pi was to provide cheap accessible processing for whatever you may need. But as innovations keep happening, things keep getting more expensive.

For an extreme example, you wouldn’t build a $3500 gaming PC for your grandparents who will only use it for Solitaire and checking their email.