this is only "EU" knowledge (so take it with a grain of salt!), but as story goes the Empire had many weapon factories - Kuat Drive Yards, Sienar Fleet Systems, etc. - and they had to decide if they wanted a superior fleet, but small in numbers or better a large disposable one, that was cheaper (and missing parts, that aren't needed)

so the deal for the fighters went to Sienar, and they got rid of every thing, that pilots in close combat aren't using (Hyperdrive!) - to make them faster (and they were fast, befor the TIE Interceptor got produced) even the shields got scrapped and the T.win I.on E.ngine was the perfect machine for dogfighters at that time ...

rumors tell, that life support was missing to push the pilots in pursuing their goal with more fury (and like Luke said, they were ment to protect convoys)