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:
<PERSON> has revealed that the <PERSON> 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.
**
<PERSON> outfit did change.
Wikipedia says:
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. 837
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 <PERSON> 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 <PERSON> and <PERSON> 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 <PERSON> coat. After briefly ditching it in "Hell Bent", <PERSON> 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 <PERSON> 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 <PERSON> at the start he would have kept something).
Of course it could be something of a midlife crisis as this article suggests. 282