Why exactly is it bad etiquette to ask to see a losing showdown hand?

[AKQJ10]

No, I don't think that's slowrolling as I understand it to be commonly defined. Slowrolling would be failing to show one's hand promptly in turn, right?

If the action goes:

Villain bet all-in
Hero call with nuts

now it's V's turn to show. For H to show prematurely would give V a chance to muck his hand with knowledge V wasn't entitled to (yet), right?

Mind you, i wouldn't make a big deal about it -- if you have the nuts and just doubled through, it looks sort of silly to quibble about showing hands. Just stack your chips and keep the game friendly; it's probably +EV.