Because a good performance in the debate might result in you cracking the top ten.
I have a great idea: if FOX (or whoever) is serious about the electoral process, as opposed to making it some kind of "pop" production, then why not allow as many GOP candidates as possible, and create a format which permits that, and then maybe winnow things down from there in subsequent debates? In other words, if the Dark Horses can't gain any traction, THEN box t hem out. But not automatically from the beginning.
Even if it's "bookish", "dry" or "boring", at least then all the candidates can try to present their positions on the various important issues.
Then, those viewers who are serious enough to actually watch the debate can pass along their impressions to their friends, family and peers.
The modern debate format seems as if it's intentionally structured to preclude even the possibility of a "Dark Horse" candidate.
Methinks the bigwigs are terrified that a Dark Horse might emerge, and none of them would want anything like that, now would they? Too much potential loss of control.
If the most important thing is ratings, "gotcha" moments, and "image", then the debates aren't worth watching anyway.
The assumption is that these are legitimate candidates.
A good many of them are just spoilers (Graham in order to split SC for JBush)
Or
fincial opportunists (rand paul, Huckabee)
or
just plain useful idiots splitting votes (Jindal, Fiorina, carson etc.)
The only legit candidates are less than five. Three if one wants to be hardcore.