It was a different time. Everything from movies to stories at the bus stop were longer, more relaxed, less rushed. www probably didn't seem like a big deal at the time.

Alternates seem... bad. Instead of 'double u', you could say 'sexta-u', but that could lead to a slap in the face. Possibly the same for sextuple-u, which sounds like someone's going to tackle you then do some strange all over you.

If it wasn't for U and V switching in the middle ages, it'd be like the french language which says 'double v', and then 'sexta-v' could be OK.