As for Lucario, he does have a higher attack (every point matters), a better STAB in Close Combat, the top priority attack in Extreme Speed, a great move pool, and he's much faster with base 95 Speed. That's why he hits like a train. Not even Skarmory and Hippowdon want to switch into a +2 Close Combat.
There.
The reason is Base Power. Stats does matter, but Base Power is way more imporant on a sweeper.
Weavile? Can't even KO Vaporeon after a Swords Dance. Reason: Night Slash it the most powerful attack it gets. And it's WEAK.
Lucario? Has lower attack but can KO Vaporeon after a Swords Dance with ease.
And those 2 pokes are almost equal (1 STAB, 1 priority, 1 coverage move and SD... Lucario can run another sets of course)
A example would be Torterra Wood Hammer X Rhyperior STAB options. Despite having 31 more base attack, Wood Hammer can still hit harder.
A even more extreme example would be Aggron Head Smash. Aggron, with a 110 base attack, hits harder than Rampardos Stone Edge, and between their base attacks there's a huge difference (110 X 165. A whole 55 difference).
That said, here's my points:
What stops Gamefreak from giving something like a 120 STAB Water Attack to Feraligatr? I DOUBT that Skarmory would survive a +2 Guts 120 BP attack from
the alligator. Or Forretress. Hell, even Cresselia would dislike that. And if you use it under Rain...
About coverage: it comes down to preference, just like you said. You could use those 2 water attacks and Return for coverage just like you could prefer more coverage.
And no move for Grass or Electric types? That's why Fera has EQ and Ice Punch, to cover his weaknesses.
Almost every electric poke is faster anyway, so the only way to damage them is with DD (Gyarados is better) or SD Aqua Jet.
So, you trade a strong STAB move for the ability to hit grasses. Preference matters.