If you read the magazine you'll know that this has been addressed by them many times.
Basically a 10 from Edge used to mean that it was 'revolutionary'. That was literally the word they used on the scoring scale. This was the scale they used to use:
1 - disastrous 2 - appalling 3 - severely flawed 4 - disappointing 5 - average 6 - competent 7 - distinguished 8 - excellent 9 - astounding 10 - revolutionary
Then, one issue, the scale changed to this deeply tongue in cheek list:
1=one 2=two 3=three, 4=four, 5=five etc
The 'revolutionary' tag had become an albatross around Edge's neck. It was easy to define games like Mario 64 and Ocarina as revolutionary because of the jump to 3D but as time progressed and gaming became more about evolution and polishing well established concepts in 3D, the term revolutionary started seeming restrictive. Amazing, generation defining games were coming and going without getting a 10 because they couldn't honestly be classed as a 'revolution'.
Basically the increased number of tens isn't an accident. They still wanted the score to be seen as significant, but they also wanted to be able to give it out more often and for it to stop seeming like it had to completely revolutionise the gaming landscape in ways that are increasingly rare as the industry matures.
You can argue whether this is a good move or not but I think ultimately they were just fed up of the grandiose nature of the criteria for a ten that had been established. Now I'd see it more as a 'best in genre' or 'one of the games of a generation' award rather than 'this will change gaming forever' award.