The ships of The Empire:

The ships only began to be built and employed in the last few years of The Republic. The Pru had devoted nearly all of their resources to creating a hyperspace battle fleet. They came up with the design for the standard OT SD and were building and commisioning them in great numbers. There were also complaints in the Senate that the Pru were developing a much larger "SSD". These were the Pru and Imperial HS warships throughout the saga.

The TIE fighters, TIE bombers, and TIE shuttles were likewise designed and produced in this period. The TIE series were the only small warships seen in the new OT. The fighters were all exactly alike- except for Vader's special fighter. The fighters were tremendously menueverable and a great rival to the X-wings (though they didn't have HD)

It was a timeless and technologically static galaxy. There wouldn't be any need to upgrade the ships if they were designed well to begin with. If the Empire had broken the strength of the HS warship powers, there wouldn't be any need for the Empire to create other kinds of warships.

The Rebel fleet in ROTJ was made up of ships all different from each other. Surviving HS warships from the CW, armed and shielded merchantmen, and local warships with hyperdrive modules pushing them. The fleet carried and launched non-hyperspace fighters.

All this was to create a strong sense of timelessness (And to keep costs down;)

Edit: The Empire had stopped producing warships in order to concentrate all resources on the superweapons. This is why some of the impoffs in the ANH briefing were so disgruntled.

Edit: There could be only one type of TIE. It could be rigged as a Fighter/Bomber with external weapons attached. Vader's TIE could be identical with only Flight Commander markings to distinguish it. We don't HAVE to see any imperial shuttles. There could be only X-wings until non-hs fighters are launched in ROTS. More resources could be devoted to developing the best possible TIEs and Xs.