I think I’ve had a workable idea to help justify why the lightspeed ramming isn’t something that’s commonly done!

Why can’t it be done normally?

It can’t breach shields.

Why can it be done in TLJ?

Because as part of their infiltration of Snoke’s ship, DJ has just “slice[d] a slit in their shields”!

We’d spoken before about adding a shot of a screen on Holdo’s bridge showing that they’re lined up, as a way of achieving something similar, but what if it shows the ‘there’s a slit’ graphic instead? The shot of the screen showing the slit is even onscreen twice when it’s first shown, slightly differently each time. Plus, the particular ‘slit’ which gets sliced by DJ (as seen in the graphic) is almost exactly the same distance down the wing of Snoke’s star destroyer as the exact line her ship slices through it when it does the ram - just on the other side, so you’d just need to flip the graphic. You’ve then got the officer on the Star Destroyer tapping the screen as if he doubts the visual error - which you could move to before Holdo triggers the attack, to remind the audience what’s up.

As for why the Holdo Maneuver can’t breach shields normally, well, I guess that’s up to the viewer’s imagination, but it certainly works just fine with the fact that every important military asset is shielded - the Second Death Star, most capital ships, Scarif, etc. (Scarif even has a mechanism where you can bypass just a PART of a shield while keeping the rest up, as ‘slicing a slit’ would have done.)

And I know what you’re thinking - in TFA they explicitly use hyperspace to come out UNDER Starkiller Base’s shields. BUT we already have the answer for that - in TFA, it’s said that that shield has a “fractional refresh rate”, which lets them inside.