First of all, the old system didn't really implement sorting for "recently changed" files. It implemented sorting for "recently indexed" documents. This was a decent proxy when we only rebuilt the index every two years. But as I discuss in the talk, we rebuild the index all the time now, so it would be pretty worthless as a sorting option. Another reason is due to the content deduplication I also discussed in the talk. When a blob is shared between repositories, what time do you store? Finally, it's complicated because of how Git works. If we did implement this, we'd like it to reflect when the path changed in the branch, which is what people mean by "when did this file last change". But Git doesn't have an efficient way to retrieve that information, so we'd have to build a system to keep track of it.
In short, it's hard to do right and easy to abuse. :-(