So, headcanon time:
 
Surely, King Sombra can’t be wearing just one attire all the time (especially since I headcanon that his first rule took a longer period of time than shown in the comics…). So, here he is getting a new set of clothes, for a less combat-ready situations, and winter themed. While I don’t think he’s all hung up on clothing, he does expect them to reflect his status and power. Fail to do that (or do your sewing shoddily) and it’s a one-way trip to the mines.
 
Considering the age and the fact that the clothes are Still hoof-sewn, clothes are Still something of a luxury and kept in good condition for a long time, mended rather than trashed. They take a long time to be made, especially with intricate details. So, while King Sombra’s wardrobe is more sparse than expected and focused on specific tasks (combat, diplomacy, court appearances etc.), there’s an clear theme of status and class.
 
Speaking of mines and slavery in the Crystal Empire, the royal seamstress is a slave as any other, but loads more comfortable position than a regular one. Any positions in the castle probably beats the mines. Even if one has to directly deal with Sombra and whatever turncoat nobles that supported him for their own benefit.