There are many, many TV shows that work very well with large ensemble casts, so the number of companions cannot be the problem in itself. However, Doctor Who is all about The Doctor. The Doctor is central to the story and the show. See how in this episode the companions were lost as to what to do when The Doctor was in prison, mainly, but not only, because they didn't have access to the TARDIS. It is also about The Doctor's relationship(s) with the companion(s) and with a single companion that relationship can be much more deeply explored than with several.
Apparently, there was a large number of Doctor Who shippers who had a particular interest in seeing a romantic pairing develop between The Doctor and Yaz, and who are now extremely upset at the John Bishop reveal.