_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 the 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_; however, see the _Chazon Ish_'s answer, below.]
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_.

_Midrashim_ quoted by the _Tora Sh'lema_ say that this is the source of "_al tiftach pe l'satan_" [which I guess means that they agree with the _Tos'fos Yom Tov_'s own answer].

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.

Abarbanel says he should make himself seem uncertain and not like he's smart. [This answer seems to me like the _Mizrachi_'s teachers' answer, but I'm not convinced they're the same.]

In _Esh Kodesh_, the Piazetzner says that the homeowner is unable to see on his own whether the _tzaraas_ is a good thing in disguise (so as to get hidden valuables) or simply a bad thing (a _nega_), so all he can say is that looks like a _nega_ to him.

Rabbi _Avraham Yitzchak Shayn_ of the Yeshiva of Staten Island writes in his book, _Birkas Ish_, that the homeowner, if he were sure that he were not guilty and that the _tzaraas_ were so he could get hidden valuables, would be glad to announce he has a _nega_; on the other hand, if he thought he might be guilty and that the _tzaraas_ were a punishment, he would only say it's _k'nega_; so the Torah is telling him that he should think he might be guilty and examine his deeds.

The _Malbim_, if I understand him correctly, seems to be saying that the problem is one like _more halacha bifne rabo_.

_Chazon Ish_ says that if the homeowner is a _talmid chacham_ then the _kohen_ will need to rule like the owner said, that there is, in fact, a _nega_, since, although only a _kohen_ can rule on a _nega_, anyone can check it. (This works, he notes, according to those who hold a homeowner can check a _nega_ on his own home, which he says is a matter of dispute among _rishonim_.) And if the owner is not a _talmid chacham_ (or if he saw it at night or under other circumstances that don't count) then (a) saying it's a _nega_ is lying and (b) one shouldn't begin talking by saying something bad.