there are many reasons:
1. 911 service
2. Perception about google voice as a full fledged provider is not crystallized.
3. other service providers provide a ton of call management features(call hunting,CNAM lookup etc) and are considered reliable with good track record.
4. One SIp Provider to another SIP provider is usually free via SIP broker(especially if you are talking to few sets of people who have their own SIP providers).SIp to SIp with same provider is usually free and is applicable irrespective of if they are in US or Not.
5. GV has not support per se for any issues you might face.
6. International termination quality is excellent with certain SIP providers. GV is yet to be proven(they just recently slashed international rates)
7. call quality varies with GV .(part of reason why people still think its nice to have but not for critical setup)
8. GoogleVoice may not be free next year, which means that there will be little differentiation.
9. Number porting issues with google voice in certain cases.
Personally, i have callcentric and GoogleVoice and let the wifey use callcentric(read mission critical
) and i use google voice.