While people usually mock the sequels (especially 4 & 5) for not having good masks for Michael, it actually makes a lot of sense in-universe. After the ‘78 murders, the original mask was likely put out of production, given the murders committed while wearing it. So it makes perfect sense that there wouldn’t be a proper mask anymore!
The reason why Michael always wears a white mask is because he feels that it's his true face (think Rorschach from Watchmen). Underneath his mask, Michael is shown to be normal-looking, however, he's a textbook sociopath with no real emotions, and wears a mask with a blank, expressionless appearance. He also wears the mask to convey a feeling of power. With the mask on, he feels unstoppable, however when he loses it, he feels weak; this can be seen when he's unmasked after killing Judith and during his fight with Laurie. He looks shocked and he stops what he's doing. This theory also explains Michael's obsession with Halloween. Remember that Michael has been locked up since he was six, so theoretically, he may have the mind of a six year old. His delusions of reality has him still thinking that monsters are real, and when he dresses up as one, he becomes one for real, as Halloween is the supposed time where ghosts, ghouls, goblins, and shapes roam the night. This is also why he scrawled "Samhain" in blood in the second film; he probably learned about the history of Halloween in school and became obsessed with it, as it gave him the idea that it was an outlet to indulgence in his sociopathic tendencies by killing someone and getting away with it; it didn't work out, since he got put in an asylum in the end.
When Michael escapes from Smith's Grove in the first film, he murders a tow truck driver in order to steal the man's coveralls. He is then shown using coveralls as his regular outfit throughout the franchise. The reasons why Michael keeps using coveralls is because since there are repair shops, gas stations, and truck stops here and there, giving him several convenient locations for him to steal a pair of coveralls whenever he needs a new pair; coveralls are usually unisex and made so one size fits all, allowing a comfortable fit regardless of who Michael mugs; lastly, the coveralls provide camouflage for Michael. When he's roaming around Haddonfield, no one pays close enough attention to him since he looks like a blue-collar worker out on a break and despite being masked, Michael's mask looks so much like a real face it looks normal from a distance. The dark coloration of the coveralls also help Michael blend in the environment during nighttime.
After watching the latest film, Halloween (2018), another theory emerges; Michael wears the same mask and coveralls because he wants to relive the feeling from his original killing spree.
In the 5th movie, we see Jamie getting rocks thrown through her window, because of what she did, and because she’s Michael's niece. Sucks for her, but think for a second. What did the people do to Michael's parents after he murdered Judith, and Laurie after the '78 murders? Laurie might have lost some of the heat given she didn’t know and killed Michael, but just imagine what Mr. and Mrs. Myers might have gone through.
