The Mishnah actually says 'Besulah', so I assume you meant to ask why the Mishnah says 'Besulah' and not 'ha'Besulah' (like it says 'ha'Ishah Nikneis', and 'ha'Yevamah Nikneis' [at the beginning of Kidushin]).

Tosfos actually ask this question. They answer that whereas the two cases in the Mishnah in Kidushin are based on Pesukim, and what the Mishnah therefore means is that both the woman and the Yevamah mentioned in the Pasuk can be acquired ..., the Besulah in our Mishnah is unconnected to a Pasuk (since the Halachah discussed by the Tana is a Rabbinical law). Consequently 'ha'Besulah' would be inappropriate.

be'Virchas Kol Tuv

Eliezer Chrysler.