1. Consoles use a controller which is built for gaming, while PCs use a mouse and keyboard. This means that there can be huge differences in gameplay on the consoles. With things like Modern Warfare, Half Life, First person shooters, are played alot more accurately because the mouse is alot more accurate then an anologue stick. Real time strategies work alot better because a mouse allows faster micro managing and a keyboard allows you to tab your units to allow you to switch between them faster. However alot of games play much better on consoles. Third person shooters involve alot of moving in cover which needs buttons, so obviously they work quicker on a console where the button is an inch away from your right thumb.
2. PC Gaming is alot more expensive. When you buy a Gaming PC, you have to pay a minimum of £500 to play any decent game on its minimum settings. In order to get it to the same quality as you would see from a Xbox 360 or PS3 game you would have to pay somewhere between £800 and £1000. Which is a pretty big difference considering you can get a pretty nice PS3 bundle for a maximum of £300.
3. The games. As I said in 2, PC gaming is expensive, but there is good reason behind that. Some games that come out on PC could not be brought out on a console. It tends to be mostly RPGs, but added onto that are games like Sims, Crysis etc. etc. Games that are either too big for a standard console to have, or that have online communitys too big for console servers to handle, or simply the controls of the games are impossible to translate from keyboard and mouse to a controller.
Hope that helps.