I think the smaller number of provinces may be to increase the frequency of field battles relative to sieges. Quite a few people have complained of "Rome: Total Siege".
Making a simple calculation:
Less provinces = less cityes, so less sieges indeed
but ... Less cityes = less places from which to recrut stacks, etc (whe not discussing that much hording)
So overall gameplay and balance may be seriously affected...

And finnaly, what about those who are passioned of historicall accuracy ?

Maybe it was an attempt to reflect that? Or the increasing importance of a small number of large cities?
Maybe... but keeping (at least) the number of provinces from 1.0 but more poorly developed, or whith lower starting population, could have been used to represent the same thing...the collapse of the Roman Empire, altough as farr as i know the great decline in population started sometimes later, around 500,600, 700... climatic changes, etc.