GT5 took six years to make because PD had all sorts of problems during the development process, and they've actually been close to cancel the project several times (that is, making GT4 the last one of the franchise). This was revealed in a long December 2010 Famitsu interview to Yamauchi.
Source:
http://www.famitsu.com/news/201012/16036986.html
That makes sense, since GT5 can't possibly be the result of 6 years of continuous work.
If they've sorted out their internal problems for once and for all, GT6 won't take as much time and might finally include free roaming capabilities (which, seeing the amount of "cruise" rooms online, appears to potentially have quite a following).