Ksav Sofer answers that if was the potential influence of Eisav that caused Ya'akov to leave home and go somewhere else to learn. The potential gain from learning with Yitzchak was far outweighed by the potential danger of being influenced by Eisav and losing everything.
He adds that the danger was especially acute because Eisav presented a facade of tzidkus. There was no chance of Ya'akov being influenced by someone who was an out-and-out rasha. But with Eisav there was always the danger that one could be fooled -- maybe he's not really that bad, maybe we misjudge him -- and then start using him as a role model.
On a completely different note, while I am sitting here by the computer my wife just showed me this article in the Economist worth a peek in case you are wondering what eruvin and thanksgiving have to do with each other.