The Torah (Lev. 14:1-7) prescribes that when a person is healed of a tzaraas lesion, a kohen is to perform, as the first step in his purification, a procedure involving two birds and several other items. The same thing is also done when tzaraas on a house goes away (ibid. 14:48-53).
Why is this procedure not called for when an article of clothing becomes free of tzaraas?
Ralbag (on M'tzora 14:49) says:
הנה מפני שנטמא הבית כל ימי ההסגר והיה בלתי ראוי שתסור טומאתו בזולת פועל כלל הנה תהיה טהרתו בצפורים לפי שאי אפשר בו הטבילה
That is, a house, like a garment, should by rights just go in the mikve, but it can't, so we do the sprinkling-of-blood ritual instead.
(This leaves open the question why tzaraas on a person requires both a mikve and the sprinkling of blood. From what he says in Sh'mini about tzaraas on a person—
והנה יטבול אחר התגלחת הראשונה והשנית להעיר כי זאת הטומאה חזקה מאד ולזה לא יתכן שתסור יחד בפעם אחת אבל תסו׳ קצתה בטבילה הראשונה וקצתה בטבילה השניה
—it sounds as though he may hold that it simply is so severe that it requires both rituals — but I don't see that he says so, or that he otherwise explains why tzaraas on a person needs both. (I may well have missed it, though. He writes rather at length there in Sh'mini.))
The Ramban on Tazria-Metzora (I don't have it in front of me at this time. When I find it I'll post it) says that the order of severity in tzaraas (from light to severe) is clothing, houses and then people. He further says that the more severe the tzaraas is the more needs to be done to become pure.
Therefore, to purify tzaraas on clothing, one needs to just putt it in a mikveh, as this type of tzaraas is the most lenirnt. Next is the house, which is more severe and therefore needs the birds (which is similar to an offering). The most severe tzaraas is the tzaraas of people, which requires all of the above (for purification) in addition to real korbanos, as this is the most severe of all tzaraas.
Just to clarify, the above is an answer based on the Ramban, but is not necessarily what the Ramban is saying.
