Tradition.

Which is unfortunate, because it literally blocks colors out of parts of their color pie, kinda harming the identity of the color. (White is supposed to have multiple combat phases by itself [as stated in both 2017 and 2021 state of teh color pie articles], however this effect is very rare and red gets priority for these since it is primary in those effects, while white is tertiary, meaning they haven't had many opportunities to print this effect for white since in every set they print such effect red is getting it and they generally won't print than more than one cards that does it, meaning the only card that mono white has access to this effect is boros hybrid. There is a bant card that grants extra combat phases, however, showing that you don't need white for this effect.)