My experience with Preply is that if you have too many tutors available on the site, tutors will put their prices down to unsuitably low levels in order to be competitive with other tutors and gain experience.
I have just seen this strategy recommended for a new tutor in the forum (although to a more acceptable £15 starting price). On Preply people often tutor for extremely low rates like $3 to $5 an hour. Such a market will lead to reduced profits for Tutorful as prices will be driven ever increasingly low.
Tutorful are presumably very aware of this potential problem with having too many tutors available on the site. Such a situation will not be beneficial for Tutorful or the tutors they work with.
It is a very difficult situation for tutoring sites because ideally everyone should have opportunity to be successful and be able to help themselves and others with their knowledge and talents. However, an excessive surplus of tutors will drive good tutors away and make it very difficult or simply unfeasible for new tutors (as is the situation with Preply).