There are many kinds of programmers’ tricks. For example, for extremely quick access to some data associated with some text, so-called hash-tables are used. They allow to find the specified line of text very quickly, without necessity of successive comparing it with every piece of text present in the table.
Now, let’s have an example: regexps could be stored in a kind of hash-table, and hence they could be accessed very quickly, without looking each of them one-by-one. If you add the possibility of setting priorities, you will have to look each and every of them, just to know if there is some other regexp that has higher priority. If there are many highlighting rules, this would become a very long task. Would you like EmEditor to wait for 10 seconds every time it has to redraw the screen? :)
Of course, I don’t know if Yutaka uses hash-tables for storing regexps, but anyway, it was just an illustrative example (and, besides, I won’t wonder if he really does).