Instead of simply travelling back in time, Nero travelled back in time AND into an already-existing parallel universe. This so-called 'diagonal time travel' has already happened once in Star Trek: the Defiant in 'In a Mirror Darkly'. In my opinion, this is the best explanation because it explains why all the technology looks completely different. Nero destroying the USS Kelvin doesn't explain that at all.
This is the worst explanation because it removes any attachment to this universe - because its just another parallel universe - if things go bad go to another one.
"all the technology looks completely different"
This is the 21st century, things have to progress for the show to be realistic, how come TOS looks outdated even though its set in the future?.
Time is a Dimension, if your going to allow time travel then your going to have to come to grips with the fact that things in the future can effect the past. (Think 2d pond and stone thrown into it, the ripples travel out both ways, not just one).