Question:
Why does everyone learn arvei pesachim with the rashbam, why not rashi?
Answer:
From what I have seen, the reason for this is because Rashi on Arvei Pesachim is a very concise commentary of what Rashi heard from his Rebbi R”I ben Yakar. (See Rashbam Pesachim 109: D”H Dhainu). The Rashbam is based on Rashi’s commentary, (the Rashbam was Rashi’s grandson) however he elaborates a little more and therefore it is clearer to understand.
