A few months ago, these miners were kind of cheap, most weren't really worth fixing, for example, S9 was sold for under $50, which means each hash board was worth about $15 when you include PSU and control board in your math, today that same $15 hash board costs $300 to get, it makes a lot more sense to repair it, even for say 50-100$.

The sudden surge in prices created high demand on repair service, this wouldn't be the case if bitcoin price was still trading under 10k, also I think many people understand this demand is temporary so expanding or starting a new repair business might not be very smart and thus the supply of these services remains constant to a certain degree.