Because if they don't do something, we would soon have over 1,000 hp machines and very high g-forces in cornering and braking.
This is what Formula One is all about, it's about creating the best package within a set of very strict rules and regulations, not about all out speed.
These cars are still very, VERY fast, just not as fast as last year, that's all.
It's better to have a safe Formula One, rather than one where we could potentially lose a driver in a high speed crash.
If the FIA didn't make a change, then they would just keep going faster and faster, and eventually someone would crash, and it would be very severe, possibly even fatal.
Another way to look at it is, they have pretty much found the maximum they can get out of the 3.0L V10's and the current regulations, so the decision to switch to the 2.4L V8's for 2006 is only natural, it gives the engineers something new to work with, and to optimize in coming years.