Waltzes are usually notated in 3/4, and the measures usually come in pairs, which fits nicely with a rotary waltz doing a complete turn in 2 measures. 6/4 is e

Waltzes are usually notated in 3/4, and the measures usually come in pairs, which fits nicely with a rotary waltz doing a complete turn in 2 measures. 6/4 is extremely common when notating Renaissance music, btw. Although the originals are often in 6/2. Or lack bar lines, so they're X/2.