Tos'fos Yom Tov to N'gaim 12:5 gives the following answers, if I read him right:
- (from the Mizrachi in the name of his teachers) Since he can't be sure it's a nega until the kohen shows up, he shouldn't say he's sure.
- (from the Mizrachi himself) He should treat the kohen with derech eretz, etc. [The Tos'fos Yom Tov includes the "etc.", thereby referring he reader to the Mizrachi for a fuller explanation. I didn't check the Mizrachi or, indeed, the other people he quotes.]
- (from the Mizrachi himself again) So the kohen not be swift to judge it as tame. [I assume this means that he'll be prejudicing the kohen.]
- (from the Gur Arye) It's not a nega until the kohen says it is, and he shouldn't lie. (The Korban Aharon asks that that's not true: the kohen says it's tame, but it's a nega tahor even before pronounced tame by the kohen. The Tos'fos Yom Tov answers that "nega" in the vernacular means a tame one, so he'd be lying to call it that.)
- (his own) Because of al tiftach pe l'satan.
Rabbi Matis Blum of Queens, in his "Torah Lodaas" weekly d'var-Tora sheet for M'tzora 5771, suggests that according to those who say there never was or will be a house with tzaraas, "k'nega" makes sense, as it isn't, in fact, a nega.
Ralbag says it's because only the kohen can adjudge it a nega. [This is opaque to me, but perhaps he means the Mizrachi's teachers' or the Gur Arye's answer?]