I know a bit of music but nothing about dance. 4 4 is a time signature that musicians find useful and common. I see no reason why what works for musicians will necessarily work as well for dancers so if they like a dance measure that’s equal to 2 music measures, that’s fine with me.
I’ll note that musicians might just count 3, 4 and then jump in on the one. Not musicians like me, mind you, but real musicians who are ready and confident. In fact, I’ve heard that good musicians who know the tempo can count 4 and have the band start playing on one. Never seen it with my eyes but heard about it from an old teacher. Naturally dancers want to save time and get to work so shortening to 5 6 7 8 just makes sense.
Jazz musicians often set their metronomes at half the tempo they want so they click on 2 and 4. It mimics the high hat in a lot of jazz. They listen to the metronome long enough to get the tempo and then count 1 pause 2 pause 1 2 3 4. The first 1 2 come in the spaces between clicks and by then they’re ready for a 1 2 3 4 where the 2 and 4 are timed with the clicks on the metronome. iamthewalrus(:3= is absolutely right that, if musicians were logical, they’d count 1 3 1 2 3 4 but, ya know, tradition. Seems reasonable that dancers, who are no more logical than musicians, would count 5 6 5 6 7 8 after adapting it to their eight count thing.