I've got a couple of cents, two actually, to put in.
One of the problems is, in my opinion, that sponsoring a major cycling team is looked at as participating in a high risk, high reward gambling game. The reward of sponsoring is, at least in Europe, the great exposure you get in the media. Just look at Belkin, they were basically an unknown company in the Netherlands, now most have at least heard their name and will recognize it when they visit an electronics store.
However, it's also a high risk game, as there's always the possibility of a major scandal developing in cycling. It does not even have to be a scandal involving the team you're sponsoring, as any (major) scandal will automatically scatter the fragile image of pro cycling. So, whether it's Belkin disconnecting, the Sky train derailing, or Europcar having a head-on collision, potential sponsors are worried of the fall-out.
Which brings us to the current situation: Get in, earn the reward and get out before something happens, just like Belkin did. They've already maximized the rewards, but feel that the rewards of continuing are not worth taking the risks.
Another problem is that a lot of sponsors are just too small a company. They're never going to provide a stable sponsorship base for a team, as they just want to maximize the exposure in the first year or so, followed by a quick exit to minimize the costs.