Why isn't Quicksilver the strongest Marvel character?
I could be wrong, but I think your confusion is coming from equating Quicksilver and The Flash. While I could certainly argue deep into the night that the Flash is by far the most powerful character in the DC universe, Quicksilver is much MUCH "weaker" than he Flash.
The Flash is usually determined to be "unstoppable" due to the fact that he percieves time at the attosecond level and can move with essentially infinite speed by drawing more and more energy from the Speed Force. This allows him to be virtually invincible and omniscient, as shown in Kingdom Come. While this is an "elseworlds" story, so it's not technically canon, it shows the theoretical upper limit of The Flash's powers - he melds with the Speed Force and is able to move so quickly he is essentially everywhere in the city at once, and is able to stop crimes before they occur.
Quicksilver, on the other hand, has limits. His top speed is "only" about Mach 5. Certainly very very fast, but not fast enough to give him the extra oomph that The Flash has. For example, at those "low" speeds, Quicksilver is unable to do his own version of The Flash's "finisher," the Infinite Mass Punch. The IMP is Flash's "trump card" against godlike entities like Darkseid or Superman, it can damage even the most durable opponent. This is an area which Quicksilver suffers in: give him a room full of cronies and he'll have a blast. Pit him against someone like Apocalypse and he's essentially slapfighting a brick wall, but really fast.
If you mean "strongest" in the sense of "able to accomplish great feats of strength", it's because "fast" isn't at all the same as "strong". If you don't believe me, try moving a bank safe by shooting bullets at it, or hooking a cheetah up to a tractor trailer. :-)
If you mean "strongest" in the sense of "overall most powerful" - the perennial "who'd win" debate - well, in the hands of a motivated writer, he probably could be presented as able to beat anyone else in the Marvel Universe. But then, some other writer motivated differently could plausibly make him seem completely outmatched by someone else. "Who'd win" is honestly more about the writer than the characters.
Because nothing in is power set, would indicate that he should be the strongest. He's not even the fastest Marvel character. He's arrogant, quick tempered and I know other characters are as well, but none of it adds anything except to make him an asshole. And we won't even touch the Quicksilver and Scarlet Witch thing (Creepy). The only interesting thing about him was being the son of Magneto. Then Marvel took that away from him. Even putting him in the MCU didn't help seeing as they killed him off pretty quickly. However I do enjoy seeing him in X-MEN universe, even though they've changed his personality.