_Tos'fos Yom Tov_ to _N'gaim_ 12:5 gives the following answers, if I read him right:

1. (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.
1. (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.]
1. (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_.]
1. (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.)
1. (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?]

_Rav Moshe_ (in _Darash Moshe_) says it's so as not to say something bad. [This, too, is opaque to me. Perhaps he means the _Tos'fos Yom Tov_'s own answer, or _en m'shivin al hakalkala_?]

Rabbi _Chayim_ Zuckerman, in his _sefer Otzar Chayim_ on _chumash_, quotes רמה״כ‎ (?) as saying that because _tzaraas_ appears on a house because of _tzarus ayin_, the owner's saying it's a _nega_ is effectively his libeling himself.