Another reason not to implement a compatible client is there's little need. Skype exists on the 3 major platforms (Windows/OS X/Linux), and what advertisements

Another reason not to implement a compatible client is there's little need. Skype exists on the 3 major platforms (Windows/OS X/Linux), and what advertisements exist are pretty unobtrusive. If Skype where to get more aggressive with their advertising, or the client quality became unbearable, I suspect there would be more effort to develop competing clients. Edit: this is pointed out in the linked slides.