Or because it's a bought article. Forbes has zero integrity and I wouldn't doubt for a second there's some sort of back-scratching/kickback/payola situation going on here.
And because, quite frankly, Hipmunk just isn't that good.
If you have a source for this I'd love to see it. The only unethical behavior I've seen at a major publication was at the WSJ, and there it wasn't payola. They have writers who blatantly distort the facts, manufacture quotes, and screw people by "forgetting" that something was off the record if it fits their spin.
Only thing I can think of that comes close is some of the trade press for industries that are dominated by a few big players. Trade press often are getting a huge percentage of their print ad revenues (and sponsorship dollars for conferences) from one or two companies, and will bend over backwards to avoid criticizing them. They'll also do slam pieces on upstart companies. But there's no explicit agreement going on, they just know where their bread is buttered.
And because, quite frankly, Hipmunk just isn't that good.