Without needing to study to learn magic like a wizard.
But what, exactly, is "cut short"?
It's not like Warlock abilities are any more powerful than Wizard ones. If anything, their generic spellcasting abilities are inferior! They never get a second 6th or 7th level spell, and unless they rest more than twice a day, they never get more than ~12 slots to spend. Admittedly, all of those slots are 5th level slots, whereas the best a Wizard can hope for is ~5 slots of 5th level per day, but the Wizard also gets 12 slots of lower level and knows roughly three times as many total spells; that is, the Wizard learns 6+2*level while the Warlock learns 1+level until level 8, then one every other level thereafter.
In prior editions, the Warlock would probably have been given some kind of actual mechanical advantage over the Wizard, to actually represent how it takes the "quick path to power" or whatever--in which case, the designers of those editions would also have applied the alignment limitations you speak of, to balance out the increased power. The problem is, the Warlock is really no better than the Wizard, except in certain narrow contexts (e.g. the Blade Pact is a much better melee fighter than a straight-up Wizard would be). So, from a design perspective rather than a fiction perspective, there is no reason to apply an alignment restriction--there is no power difference.
The same argument applies to the Paladin. In Ye Olden Dayse, the Paladin was pretty much a Fighter with extra "toys" who did not become a landed noble at high levels. The extra powers were straight-up above and beyond what a normal Fighter could do. Thus, it makes sense that you would give an alignment restriction--their increased powers "come from" a devotion to a higher calling. But the 5e Paladin
isn't just a Fighter with more toys. The two classes are now made to be roughly on par with one another. Whether you call it "balanced with each other," "at parity," whatever, they're meant to be at approximately the same power level. That means the restrictions no longer make sense from a design perspective.
From the fiction side, there's long been a debate about alignment descriptions and whether it is sensible to have an X that is not Chaotic Fashionable or whatever
P). Concepts like the Blackguard in 5e, particularly given the Oath of Vengeance, may make just as much sense with a fallen Paladin as they would with one that is not fallen, but just Evil to begin with. An Oath of the Ancients Paladin is probably going to be Good, but the mentioned tenets of the oath could work regardless of how a person feels about social norms. (For example, I could see one fighting to protect graffiti art, as long as it was actually beautiful, laws and social norms be damned.)
So...yeah. In prior editions, there were often reasons which justified the inclusion of alignment restrictions. A lot of those reasons no longer apply in 5e, so there's no longer much point to the restriction unless one is aiming for specific flavor. And "specific flavor" stuff has been, in general, left to the DM in 5e.