Leotards are tight and act like a second skin and the teacher can see your movements properly and would be able to correct them if need be. She/he wouldn't be able to do that well if if you were wearing baggy shorts or tracksuits would they. You may also want to add that Gymnasts wear leotards for much of the same reason for example the ladies wear very tight. This is leotards be it in velour or shiny lycra or even a shiny lycra and velour combi because one of their apperatus is the uneven bars and you gould imagine the young female gymnast swinging round and round on the bars and flipping from one to the other in tracksuit and maybe even baggy shorts and the injury they could do to themsellves. So thats why the ladies and the guys as well wear leotards for gymnastics so the could perform with ou fear of their clothes falling off as it is tight and clings to them also for the judges to see all the correct muscle movements in the gymnasts body.
Hope this helps[/i]