But that makes the assumption that every one of them is a legitimate candidate with a shot at winning. If they can't demonstrate that they have a base level of support, why put them on the main stage? IIRC, the cutoff level is around 4-5%. If someone's candidacy is so uninteresting to the voters that they can't generate even that, then why bother to have them there taking time away from the candidates that the voters have indicated they're interested in hearing from?
This is where, much as James Brown notes, money factors in. I don't think the poll numbers at this point for the Republican candidates actually indicate who their voters are interested in hearing from. The numbers reflect who has the largest name recognition and/or the largest pockets for advertising.
Moreover, just in the interest of fairness in elections (I know... expecting fairness from Republicans) they really need to allow every candidate equal time in the debates.
There are 17 candidates running. Have two-part debates with the candidates in each chosen at random. As the field narrows when candidates drop out, then consolidate to one debate.