Closing the top button takes extra time longer than other buttons. It is bittul zman. Those extra few seconds multiplied by thousands (Kein yirbu) of yeshiva-leit adds up to a significant chunk of time that could be used for more constructive purposes.
BTW – this is also why many masmidim wear slip on shoes, to save the time of tying and untying laced shoes, (even though they are mivatel kiyum of a halacha of tying the left shoe first).
However, besides the b’maizid unbuttoners, there are also some anusim, who simply can’t close the top button.
I know of some masmidim who wear clip on ties on shabbos for this reason (and also because they are choshesh of making a kesher on shabbos).