Answer

For several cumulative reasons: Maimonides deals with Kodashim and Taharot. He addresses and spells out all the Jewish laws, so there are contradictions and a demand for consistency in his work. He does not explain his method, and that creates contradictions and leaves room to propose our own explanations.
Rashi and Tosafot do not have to be consistent, because theirs is a local commentary, committed to a particular view and to the specific Talmudic passage under discussion, not to halakhic rulings in a general coherent system. Raising a contradiction from one Rashi to another, or from one Tosafot to another, is not very difficult and does not necessarily require an answer. They also explain more of their method.