Having just read the story myself, my presumption was that George knew Lennie couldn't survive on his own for any length of time. The mob was fixated on finding Lennie and the police had been called.
Curley said, "You go in Soledad an' get a cop. Get AI Wilts, he's deputy sheriff.
So there will be a manhunt on from two directions, which won't be given up easily until Lennie is caught. George needs to try and stay with them to demonstrate his own innocence. In the meantime, it's quite possible that Lennie is so childlike that he will be unable to hide successfully in any case. Thus, a diversion is unlikely to work and will take considerable time to pull off.
Several times in the Story, when he is feeling guilty, Lennie talks about going to live in a cave somewhere so as not to be a burden to George any longer. However, to the reader, it's obvious this is an empty threat. Lennie's faculties are so impaired that he can barely even remember simple instructions and he has no real understanding of the consequences of his actions. Hiding alone he would be unable to even feed himself, possibly even to get water or shelter. George makes this plain at one point.
"Yeah' How’d you eat. You ain’t got sense enough to find nothing to eat."
So even if George could lead a successful diversion, the likelihood is that Lennie would suffer a miserable death while he did so. Better, George reasons, to have him die quickly at a moment of happiness.