Why Quicksilver's Costume In The MCU Makes No Sense
The Marvel Comics character of Pietro Maximoff (aka Quicksilver) was first adapted to film on two significant occasions, with two well-known actors playing the role — Evan Peters and Aaron Taylor-Johnson. According to IMDb, Peters' Quicksilver first appeared in 2014's "X-Men: Days of Future Past" alongside most of the cast from 20th Century Fox's X-Men film franchise, set distinctly apart from the still-growing Marvel Cinematic Universe. Meanwhile, Taylor-Johnson's Quicksilver first appeared only one year later in the second Avengers film, "Avengers: Age of Ultron." Taylor-Johnson starred as Pietro Maximoff alongside Elizabeth Olsen's Wanda Maximoff (aka Scarlet Witch), as two orphaned Sokovian siblings who were experimented on by HYDRA, thus giving them their superhuman abilities.
While Wanda's powers are significant in their own right, Pietro was also blessed with amazing powers, including superhuman speed, extreme force generation, accelerated perception, increased metabolism, and improved thermal homeostasis. Basically, Pietro is extremely fast and his body is built to handle that speed, similar to DC's Barry Allen — also known as The Flash (Ezra Miller).
However, Taylor-Johnson's stint as Quicksilver was cut quite short due to his death in "Avengers: Age of Ultron" when he quickly grabbed and shielded Clint Barton (Jeremy Renner) and a small Sokovian child from bullet fire. The bullets were fast enough to get him, but if Maximoff had a costume built for force and speed like Miller's Barry Allen, he may have had just enough protection to walk away. So, why didn't he?
Pietro Maximoff only wore what appears to be athletic clothing
Although it could be argued that Aaron Taylor-Johnson's Quicksilver hadn't developed enough in "Avengers: Age of Ultron" to have a protective supersuit that complemented his superhuman abilities, what he ended up wearing during the film's final battle gave him virtually no protection against outside forces. The athletic long-sleeve he wears along with his form-fitting pants must have some form of friction and flame resistance in order to not burn up when he does move incredibly fast, however there isn't much armored plating there. If Pietro were to simply trip and fall at a high speed, he would likely be incredibly injured — even with his increased metabolism and healing factor.
At the very least, one would suspect that some form of kevlar could be woven into the clothing, giving Pietro at least some protection. Even Tony Stark (Robert Downey Jr.) could've helped with that one, or at least created some kind of on-the-go suit for Pietro and Wanda. Either way, the outfit of choice is incredibly impractical. However, the assumption from HYDRA, The Avengers, Wanda, and most likely Pietro himself, was that he could probably move faster than anything that might come his way — though if that's true, they were all tragically wrong.
Taylor-Johnson's Quicksilver did not make a return in "WandaVision" to the surprise of many fans, as Marvel confusingly and playfully included Peters as Quicksilver instead. It is unlikely that Taylor-Johnson's Quicksilver will return, but anything is possible with the MCU.