Let's look at how (simple) combat works in (classic) D&D variants. There's an attacker of a certain level and a defender with a certain armor class. The attacker makes a roll, modified to take into account the attacker's physical capabilities as well as the situation the attack is made in. If the attacker hits, the attacker rolls damage which the defender subtracts from hit points. Not very complicated, is it?
Note that there's one roll for the attack and one roll for damage if the attack succeeds, that's it. Specifically, the defender does not make a roll to see if the successful attack can be evaded somehow. (And neither is there a roll to avoid some of the damage.)
Now let's look at how (simple) spells work in (classic) D&D variants. There's a caster of a certain level and a target of a certain level. (There are spells in which one or the other level doesn't matter, but that's besides the point.) Provided the caster doesn't get distracted while casting, the spell will be cast successfully. Now the target gets to make a saving throw against the spell. If the saving throw fails, the full spell effect applies to the target; if the saving throw succeeds, only some or none of the effects apply to the target.
Note that there's again one roll, but this time the roll tells us whether the defense was successful. (There may also be a roll for damage made by the caster, ignore that.) Specifically, the caster does not make a roll to see if the spell worked or fizzled somehow.
But now look at surprise. Surprise! There is both a chance to be surprised and a chance to surprise someone else. Details vary by which version of D&D you're looking at, but they all seem to maintain that both sides get to roll for both things. Having both sides roll dice in this situation but not in the other two situations seems rather odd. (I have no problem with the fact that the first two use a d20 and the third uses a d6, that's not the point.)
There are other places where a "let's use two rolls" mechanic has crept into D&D, for example when trying to disarm someone: You have to hit, but then the defender gets a saving throw to avoid dropping their weapon. There are also places where two rolls actually make sense in a way, for example for spells that require touching the target with a successful attack.
Now some people may just not care and some may say "different mechanism for different tasks are a-okay" or something close to that. But for myself, I would prefer a clear line throughout the whole system: Either use two rolls consistently, or use a single roll consistently, but don't jump back and forth. Opinions?