Because compared with manual flight it's a lot slower, takes a module slot, and makes you more at risk of interdiction (even by the NPC sloths) - while not doing anything you couldn't do yourself.Well, its pretty clear now frontier don't like supercruise and are building into the game ways out of it.
Was just thinking for the new supercruise assist function, say a new player follows along and gets used to playing elite in the new world. I suspect its going to be common that because there's actually no or very minimal incentive in turning off supercruise assist... people will get used to having it on permanently. If you "grew up" with it, then compared it to turning it off, why would you?
That's not me saying it shouldn't exist, because being shown how supercruise works by the autopilot is probably a pretty good way to pick up the basics for yourself - but the beginners who have some curiosity about game mechanics are likely to figure out faster approaches are possible without it.
(Same as in FE2/FFE - the autopilot was pretty effective, but you could get to your destination a lot faster by turning it off, flipping 180 at midway, and using your main engines to slow down instead)
Think of it like turrets - they're nowhere near as powerful as gimballed or fixed weapons, and a fair bit more expensive, so there's a big incentive to learn to aim, and people do ... but people don't have to and even for experienced players there are some uses for it in specific circumstances.