If you look at the frame data of all four ledge options, the first vulnerable frame of every character will be as quick as 13 or as late as 35. There are four exceptions with Palutena and the three Mii characters having a first vulnerable frame of 44 on getup for some reason. But you can't afford to be late, because the opponent can act just a few frames after their vulnerable frame, except for ledge roll which has something closer to 20 endlag before they can act. Ruling out the fringe case of Paltena/Mii's ledge getup, a move with at least 23 active frames covers all of that with no need to read. But there's more you need to consider. You have no idea when the opponent will choose an option until you notice their subtle animation. And there's mixups involving their ability to wait, or let go and jump onstage. So to actually cover all ledge options you need something with enough active frames, and with as little startup to cover your reaction timing of noticing they input something. Rock Smash has 23 active frames (most of it is residue damage), but 24 startup. It's impossible to see ZSS's animation for ledge jump and hit her when her first vulnerable frame is 13 and the move your using is active on 24. You would have to input rock smash pre-emptively and hope they don't decide to wait at least a little while to notice the move you're doing and their opportunity to punish. In other words, it's a read. We don't want to read. Finally, spacing is another restriction. Ledge roll has the opponent vulnerable beginning at about the end of the distance they travel, the other three have them vulnerable right there at the ledge. That's a lot of horizontal space to cover. Going back to the Rock Smash option, you can't space it in a way that covers both ledge roll and the other three at the same time, you'd have to make a choice here in spacing - another read.
Flamethrower will have more luck when used pre-emptively. It reaches far enough, so far you can't be reasonably punished from their position, and the flames are active for long enough. The only problem is that flamethrower won't do much for you. Doesn't lead to a kill, would only singe them for 2 or 3% guaranteed damage before they can get over or grab the ledge again, and eventually peters out or autospaces to push you too far from the ledge. It's not a great option, but it's the best choice for strictly guaranteed damage. A charizard main might instead prefer to hang back and prepare for a dash grab on ledge getup, attack, or roll. Jump is the least reasonable to grab, but they are putting themselves in a position above you, which is still advantageous in its own way.