Well, anyone who honestly thought going into the series that the NX-01 was going to look LESS advanced than the NCC-1701 had to be smoking something pretty strong. NO ONE would have watched Archer and the crew flying through space in a cardboard box (the only thing I can think of that would look less advanced than Kirk's Enterprise!) so it's pretty much a given that Archer had to have a sexy looking ship.
I think the reason behind this (prob not what they were thinking at the time they made the set design, but still...) is that by Kirk's time the Federation had been around quite awhile, so that society was lulled into a sense of security.. therefore technology advanced slower... I mean if you live in an Utopic society, you don't really need anything more then what you already have... I mean, the reason for any form of progress is to fill a need in the day to day lives of members of a society... but if the members of that society have little or no needs, the technical progress dries up... look at such examples of the Roman Empire... they had made many great leaps forward in their Tech during their conquest of the known world... but once it was conquered and Hadrian built his wall... the society progressed little on their own until their eventual downfall...