You aren't allowing them to be themselves- Everyone has their own style- if you don't like it- as long as they do- that's all that matters. You should care more about what's going on under those clothes not what they have on their back (as long as it's clean).
It's what's inside that counts
It's their job- not yours- They are an adult- therefore they should know how to dress by now. Let them do it- they aren't a child nor do they need to be treat like one.
It's too time consuming- They may put up with most things you buy for them, but if you get them something they truly hate you're the one who will have to take time out of your day to return it.
It might knock their confidence- If they think you hate the way they dress; this self-consciousness might extend to other things too- making them very wary of your opinion of them. This isn't conducive to a healthy relationship- they should feel that you're proud of them not ashamed.
It might give the impression you are trying to fix them- With one area of their being at a time. They are not your project- they are your partner- if you think of them as a before and after- you shouldn't be with them.
You might like to mix it up- And buy diffent styles of clothes each time. On the other hand, they might like to stick with the same brands, styles and colours. If they are comfortable with a more predictable wardrobe- let them be and go and buy yourself some new stuff instead.
Their friends will make fun of them- Like it or not- it's not ideal that your partner buys your clothes. It can feel more like a parent/child relationship than a romantic one.
They might lose respect for you- If they entered into the relationship thinking that no one of that mattered to you and suddenly your focus is all about appearances- they may believe you've turned shallow. That's not what they signed up for.
They are not your play thing- Dressing something is what you do when you're a kid to your dolls and action figures, not a grown up person your sleeping with.
It's a slippery slope- They may succumb to wearing what you want- but what else are they doing to keep you happy that they really don't want to do? Think about it.