Another reason is 'experienced' developers can sometimes be difficult to work with. They can be arrogant and bull headed, 'my way or the highway' sorta attitudes. Negative to others with new ideas, etc. Teams that already have some vets on them like working with Jr folks precisely because they are moldable.
I've seen that before, it has some good points. I don't think an experienced person has to - or always does - get like I described. But it does happen.
The question was "why higher less senior people?" My answer, and perhaps I wasn't clear, was that for your first 1-5 devs, you should definitely higher senior...but once you break past that then your senior devs - you know the ones who launched all that boring technology - then want Jr devs to mold into doing things the right way from the start.
