1. Obviously they haven't bundled any one game on a semi long term basis to get it to 5M, just to clear that one up.
2. Games are a hit/miss affairs and theres a bit of luck involved. The luck just hasn't gone their way. Microsoft has had some more luck here but they only have 2 franchises over 5M mark.
3. Critical appeal doesn't always translate into game sales, see Nintendo.
4. Their ensured 5M+ seller hasn't released yet whereas by the time GT5 releases Microsoft would have 3 5M+ Halo games + 2 5M+ Gears games and maybe something else on the way for 2010.
5. They faced severe multiplatform competition from game developers who had been developing for longer on present systems. They had to release Resistance 2 and Killzone 2 after Modern Warfare and Uncharted 2 after Modern Warfare 2. On the other hand Microsoft games tended to hit their marks first such as Halo 3 before Modern Warfare and Gears of War before anything else of note. This let Microsoft establish the mindshare for their key franchises and ensure future releases can justify large expenditures on budget and advertising as well as ensuring a good deal of public attention.
6. Fragmented audience. Microsoft is boring in that they nailed both the shooter and RPG markets early. This means they can continue to cater to this strong audience on their system whereas Sony bought in a lot of people on name recognition alone with a variety of tastes which they cannot as easily satisfy. Arguably they only have a solid audience in the racing genre and they only have two releases of note planned for that genre in Mod Nations and GT5. This is due to Sony not seeming to have a particular direction in their release strategy.
7. Their releases haven't yet been good enough to deserve 5M+ sales. They have been flawed in some way or missing key features which narrows the target market considerably. I doubt that even for the PS3 audience, graphics pushes nearly as many sales as better gameplay and coop for certain titles.
