Sorting can take a lot more time than just updating scores
So, a user earns some reputation. To update the displayed reputation gain, the website only has to update a variable or two somewhere, one of which is then accessed when you look at that page.
To update the order, however, takes significantly longer. I don't know exactly how they store their stuff, but it could easily take a dozen orders of magnitude longer to sort the user list rather than just update one user. This is especially important on sites like SO, where there are 4.8 million users.