The clothes reflect how the Doctor is changing throughout his run. For example, after the Christmas episode of season 8 this article states that the Doctor is lightening up a bit:
Steven Moffat has revealed that the Peter Capaldi's Doctor will return to season nine much different than he was in season eight.
“We’re not bringing him back exactly the same as we left him, at all. I think that was already evident at Christmas,” he explained. “He’s left some of the burden of being the superhero of the universe behind.”
So considering he's not going to be showing off himself as a hero he may have felt that he didn't have to bother too much about whether or not he looked good.
Matt Smith's outfit did change.
The Doctor's initial outfit, chosen within the narrative of "The Eleventh Hour" from an array of clothes found in a hospital, is a brown tweed jacket with elbow patches, bow tie, suspenders, black trousers and black, ankle-high boots. He has a fondness for bow ties and fezzes, proclaiming them as "cool."
The details of the outfit vary, switching from a braces and bow tie combination in red to the same in blue. In later stories, a green military coat makes its way into the Doctor's ensemble, occasionally replacing the Harris jacket.
On other occasions he does not wear a coat and wears a bow-tie and dress shirt. After the Doctor loses his original tweed jacket in the episode "Flesh and Stone" he replaces it with a plain brown jacket which he wears until the Christmas special "A Christmas Carol", which aired the Christmas after the eleventh Doctor's first series. In this episode the Doctor wears his most famous "Prince of Wales check" tweed jacket which he wears, with some switching to a green military coat, until parting ways with Amy and Rory in "The Angels Take Manhattan".
As to the red coat, Wikipedia says:
In "Face the Raven", the Twelfth Doctor sports a red velvet variant of his Crombie coat. After briefly ditching it in "Hell Bent", Clara comments that it made him look more 'doctory'. At the end of the episode, he dons the red coat in honour of her, and is given a new sonic screwdriver by the TARDIS.
So depending on how he remembers Clara in season 10, he may keep it as a reminder (although he hasn't done this with the other Companions as far as I know - and you'd think that because he was so attached to Rose at the start he would have kept something).
Of course it could be something of a midlife crisis as this article suggests.