Why is there so much price disparity for a house painting job?
There are many reasons why a job would cost the contractor more or less, which obviously affects the bid price:


Time spent ensuring an excellent application of the finish
Time spent being careful around plants, yard furnishings, wall features, etc.
Special equipment needs unique to your project
Insurance coverage
Employee salaries
Satisfaction guarantee
Finish warranty


More significant, though, is the level of demand for a particular contractor's work:


Brand awareness and familiarity
Reputation
Marketing prowess


The latter group of factors probably has more impact. A company that has a few dozen or a few hundred employees and is a top contender in the yellow pages will get more calls. Therefore, they can charge more. A little one-man operation may have to scratch harder for jobs, and therefore is willing to undercut the competition. 

The inverse can also be true. A small shop with a reputation for excellence can develop an outsized reputation that puts them in demand greater than their ability to fulfill. Now they can name their price and walk away from jobs they don't want. This isn't unique to painters, of course. It's the fundamental law of commerce. 

I'd select the lowest bid from a contractor that meets your standards for reputation, personal demeanor, etc. If a guy acts like a goof and slobbers tobacco juice on your driveway, that's a red flag. 

Also note that 15 years probably isn't a reasonable expectation for the life of paint over wood. Even the best paint will fade significantly in that time, and the nature of wood means that the bond will be broken down by expansion and contraction, warpage, etc. Five to eight years is probably more appropriate.