Same reason everyone else has problems competing with Apple iTunes Store. They make half assed windows and Mac solutions for interoperating with their core products.
Step 1) Make a cross platform library for audio and video communication that compiles on Windows, Mac and UNIX systems.
Step 2) replace software codecs with platform specific hardware codecs
Step 3) build an awesome UI for Windows AND Mac. Make sure you have a good Metro UI as well these days.
Step 4) make sure there is encryption... No SRTP crap. ZRTP or something better.
Step 5) make it integrate with PBX systems via SIP or Cisco ATA boxes.
Step 6) add cheap or free calls to classic phone networks
Step 7) spend a bunch of time figuring out why the heck you just invested $10,000,000 and two years in the development of a product that has no reasonable means of producing revenue.
Step 8) recognize that Skype technology is core to Microsoft Lync and FaceTime is core to iOS and get a trip and realize that there is no practical market share left for another player
Step 9) laugh you ass off at Cisco for pissing away a ton of money on Tandberg only to be almost entirely obsoleted by Skype and Lync and tablet devices.