For single digit hours, we use the native Korean numbers predominantly. As it goes higher past 12, both native Korean and Sino-Korean numbers are used. For "24 hours", 이십사 시간 is already winning over 스물네 시간, although the latter is also used (we say 스물 넷 when it's by itself, 스물네 when it comes before a noun/count word). As you go higher, Sino-Korean numbers become dominant ("1주 사십 시간 근무"). The native Korean numbers are mostly used for small figures commonly encountered in daily life. As the figures get bigger they give way to the Sino-Korean numbers which are shorter and more convenient.

Yes, It's a very very reasonable question I guess. I don't know why we call it like this. but this one just became as a proper noun. only my opinion though.