[elai]

Skype has a big 'it just works' factor. With most sip and other products you had to deal with arcane stun server setup, and other configuration that would confuse most people. And on top of that the quality was significantly better than other options at the time such as msn and others. The combination of higher voip quality, ease of use, and no tech wizardry required (and a lot of marketing after they got rid of the tech hindrances) it went off.

[MC27]

Indeed. Whenever I looked in magazines targeted at casual internet users over the past decade, Skype was always the service recommended as "VOIP". It works through practically any network problem, which makes it ideal for hotels and from use inside corporate networks.

The quality is high, it's highly accessible and it doesn't have an equal competitor. Even the instant messaging services that do voice and video are poor alternatives. It has also been used for several years by feature phones and smart phones - some as their main selling point.

Essentially, its a pseudo standard used by most people.

[sunny_s]

Yes, i totally agree with all of that. They had a huge 'first mover' advantage. Just that there's no tech-wizardry involved in there closed protocol.