It's not really that surprising. At some point the established players ossify and are unable to innovate when the playing field changes.
A new thing comes along without this problem and dominates the new playing field. For Teamspeak and Ventrilo the field changed in that it became possible to host the voice servers themselves, rather than the 'customers' hosting.
A new thing comes along without this problem and dominates the new playing field. For Teamspeak and Ventrilo the field changed in that it became possible to host the voice servers themselves, rather than the 'customers' hosting.