The reason more people don't do it is because:
A) it's difficult. Since the vast majority of miners didn't get in early, they weren't able to get in on the "gold rush" that enabled them to ramp up hardware in accordance with their output. When mining first started, you could find blocks with a decent CPU. Since then, dedicated rigs have become almost a necessity if you plan to make any kind of money at all.
B) Electrical efficiency. Since the recent decrease (reduction from 50 bitcoin per block down to 25 per block) and the rise in popularity of bitcoin (thus rise in difficulty of finding a block.. it scales naturally), the "cost" of finding blocks has gone up. By that I mean it requires more raw computational power to mine, raising the bar on both the hardware needed to be relevant AND the amount of electricity consumed during the process.
There are a lot of calculators out there. They will give you some insight into how much you could make (or very likely lose) depending on your local electricity pricing.
