You’d like to think it’s something like command of the issues or the ability to inspire, right? Maybe it’s just plain old cash. From the Center for Responsive Politics:
Continuing a trend seen election cycle after election cycle, the biggest spender was victorious in 397 of 426 decided House races and 30 of 32 settled Senate races [in 2008]. On Election Day 2006, top spenders won 94 percent of House races and 73 percent of Senate races. In 2004, 98 percent of House seats went to the biggest spender, as did 88 percent of Senate seats.
Of course, cash may be correlative instead of causative. That is, candidates that are better qualified, better on the issues, and better able to inspire voters raise more money than their opponents, and then go on to win.