INMHO They are leaving for two reasons, three if you count they are not winning.
1. In season testing has been banned, so a slow car cannot be made faster quickly by throwing money at it. This has been, historically, the way the big teams have been able to win championships. Witness McLaren taking to Hungary to improve a dog of a car. In years gone by they would have had a competitive car after the first few races. It is no coincidence that the early season lead car, Brawn aka Honda last year, was the car that Honda decided to concentrate on early in 2008 as their 2008 car was such a dog. McLaren concetrated on the 2008 car to win the title for Hamilton so the 2009 car was underdeveloped.
2. The muppets at FIA have made the whole circus into a farce over a number of years. Some examples:-
a) Ruling in favour of Ferrari at times in the face of all justice. Montoya was penalised for passing Schumacher twice in one season, unsurpisingly he went back to the USA, where he had been the cream of the Champ Car drivers before coming to F1. Then they set up a committee to enable overtaking!!!
In attempting to limit costs they are moving to a Formula more akin to Indy Cars and Champ Cars where anyone can buy the chassis, engine and tyres of the winning team and start their own team. Fine if that's what you want to do but it ain't F1! Forumla One has always been prototype racing, yes people bought engines or designed their own but the chassis was the teams' own work.