why did the VOIP Revolution end?

SIP might be an open protocol, but networks were built to be proprietary and have not been bridged together. Most telecom services still communicate with each other via public switching, meaning that the wonderful possibilities that SIP might enable are limited by the capabilities of the plain old telephone system.

I think one of the main reasons was that ,companies like Nortel (NT), Siemens (SI), and Ericsson (ERIC) rank among the top VoIP equipment vendors today, not startups. Technologists underestimated the sway and leverage that the traditional vendors held over their customers.

In my oinion ,the anticipated period of "creative destruction" came in 2001 with the smiting of the competitive local exchange carriers (CLECs) and long-distance competitors, who had not yet even had time to embrace VoIP, by predatory pricing from the incumbents. It continued with the shift from fixed voice lines to wireless phones, as evidenced by the drop in landlines. More recently, the blame has been turned toward the VoIP equipment vendors that began the revolution in the first place.