Rashi’s line-by-line explanations of the entire talmudic corpus opened the book of the Talmud in a way that was previously unparalleled. However his commentary was a local commentary, and he did not seek to analyze each line of the Talmud in context of the entire talmudic corpus. Rashi’s Talmud commentary focused on the local discussion, and he chose explanations that presented the local passage with the most clarity, even if this required ignoring a relevant discussion in another tractate.
Aryeh Leibowitz, “The Emergence and Development of Tosafot on the Talmud”, Hakirah 15 (Summer 2013), 146.