because nobody cares what their taxi driver looks like. sex workers are not generic and the customers do not want generic services. there needs to be some degree of personality, negotiation, and so on. this requires profiles and custom ads. sites that provide these tools already exist, and they're frequently subject to things like this: you'd be a fool to try and make this happen in a walled garden like an app store, much less put your signature on an investment deal. there's no "silk road" style venue for prostitution because sex work is inherently personal. you can't fellate someone over the internet, yet. things that can be done over the internet, like cam shows, are mostly legal and already have aboveboard venues like Chaturbate.
Pragmatically, sex workers are already subject to people constantly trying to rip them off, and bitcoin usage/intense anonymity of clients is a step in the wrong direction, from their perspective. A rating system for customers and workers would be fantastic, but you'd be really hard pressed to get a client to sign up for anything attached to their name, and there are already venues for workers to advertise and some are able to network and get feedback about potential clients. There have been alternatives to CL for a long time, but CL was used mostly be clients who didn't want to communicate with anyone except for the worker and potentially have any information about them recorded. And of course, seeing as how the Silk Road drama played out, perhaps it's not the best model to emulate.