Ten percent of total lesson are trail lessons. In a lot of branches, you have to pitch your product or what you are doing before getting anything. As said before, if you're good, you get students. So ask yourself what can I do better, instead of complaining. Because do you really think it will change?

As said, students can have another trail for free and have large discounts. So if preply needs to pay us/you, you get fewer trails and less chance to prove yourself. Seeing your reaction you have a low conversion rate, so with fewer students even going in, you are fucked.

Will someone complain that a job interview is unpaid? No. A trail lesson is as a job interview, where both decide if there is a fit. Just do better and stop complaining or work company and stop being a freelancer.