Likely because it's significantly more expensive to call a UK mobile than a US one. I believe that they announced free calls until the end of the year. Maybe they'll roll it out in the UK at that point but expect to pay for it.
Twilio: $0.02/min to all US and $0.03 to UK landlines or $0.32 to UK mobiles -- yes, 16x the US rate.
Voxeo: $0.02/min to all US and UK landlines or between $0.14 and $0.35 for mobiles.
Google wouldn't use Twilio or Voxeo for this kind of service so it gets cheaper elsewhere -- especially on their scale -- but it shows the relative difference.
UK rates really are a crapshoot. Voxeo and Twilio must just be averaging the rates for mobiles. The rates I get at Flowroute for UK mobiles vary wildly: https://www.flowroute.com/services/rates/U/
... and the recipient also pays for "received airtime". Whereas, in most of the world outside the US, mobile phones get a different area code, and it is the caller that pays for "airtime".
To be fair, its not that bad. When I lived in the states unlimited nights and weekends started at 7PM and lasted 8AM. I also had Sprint SERO which was completely unlimited at $99/mo. + $25 in taxes.
When I moved to France, unlimited and unrestricted calling plans didn't exist yet and the talking time was pretty measly. Even now their nights and weekend billings were awkward 8PM-Midnight, Saturday afternoons - Sunday Midnight. They gave you a feature that was practically worthless.
This explains why SMS took so long to get off the ground there. A message cost $0.15, which is about the cost of a voice call, and unlimited messaging plans were $15-20 in addition to your voice plan. That money could be better spent on an even bigger plan with 1000 minutes. That and T9 sucked as a keyboard interface.
Eventually, I hope the rest of the world gets real
There are a handful of internal users w/ UK numbers but cost has been the concern since it's significantly more expensive to acquire numbers in the UK vs. the US.
