More money, possibly a better shot at a title, program with more history and standing, etc.
Lots of coaches also like a challenge and get bored. Once they get to as high as they think they can at one place, they want more.
Take James Franklin at Vandy; the reality is he probably has 2 scenarios that will happen, either he'll get Vandy to a 10 win season and a New Year's Day bowl like the Gator Bowl or maybe the Cotton Bowl and that will be as high as he can take them, or in 2-3 seasons he'll start to lose again and won't be able to sustain the success he's started. If it's the first scenario he's going to look at that as "This is the best I can do here" and probably jump ship to a higher profile job while he can. If it's the second scenario, then he eventually fades into the oblivion of Vandy coaches, albeit the most successful one in a long time.