I have always assuned it is that it looks like a V - with intent, there is a W model that covers the Verification and Validation cycles as two V running together.
I may actually have to go and google its origin as I for the life of me cannot remember it
Agile Testers of the World UNIT!
It is just a simple convention to align the input requirements at different stages of the SDLC with the output validation processes at the later stages of the SDLC. They could easily have built a half-moon or rainbow model with many arches, I suppose, but folding the SDLC "timeline" to align complementary processes was a much neater picture and it caught on. It has become the standard, and ITIL has tried to pretty it up and make it ever so thorough, but it is basically the same as what it was when it started out.