The biggest reason is that there's no real upgrades for people who want to replace their 3-4 year old machines. I'm currently running a 13'' MBP mid 2010, C2D 2.4 GHz, 128 GB SSD, 500 GB HDD, 16 GB of RAM and i can't see a clear upgrade.
MBP 13''? Why? Same screen, a little more processing power and SATA 3 for the SSD.
rMBP 13''? Again, why? Ok, much better screen, a little more processing power, but no 500 GB HDD and no 16 GB, 8 GB is the maximum.
This is fine for now. And will probably be for a few more years because it is not a numbercrunching machine.
And that is hurting Apple, and not just Apple but the whole PC industry. My desktop is a Hackintosh, i7-920, X58 motherboard, 24 GB of ram, all that, aside from RAM is from more than 4 years ago. Again, why upgrade? If it continues to function, I have no reason to upgrade for another 4 years.
a) how did get 16GB RAM to work with Mid-2010 Penryn MBP?
b) current 13" MBP has more than twice your MacBook's processing power. However, if it's enough for you, there's obviously no reason to upgrade, but still it is not "little more processing power"
Even though people may not NEED to upgrade they computers they do, because they WANT to.