1. Its smaller and thus can cut closer to Mass shadows than a big ship can
2. Its bascily 1/2 an Enginee with a little cargo capability

3. Its been ENDLESSLY tweaked by the Wookiee and Han for years now
Sure you could put a .5 Hyperspace Drive in an ISD but because of its larger size it would have to move farther around other shadows than say the Falcon does and it would be large and more complex, making it harder to *tweak for that extra 4% power-increase that the Falcon has