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Likely that the tube in the fork was only reamed out enough for the stem it came with.
@yak, more likely - in my opinion - is that you're hitting the butt in the steerer tube. They are heavier walled at the bottom to provide more strength where the crown attaches. The extra wall thickness can't go on the outside as a constant od is required so it goes on the inside. The headtube looks very short so consequently you don't have much of the thinner walled section of steerer into which to insert a stem.
Depending on the steerer used the difference in internal diameter could be as much as a couple mm.
I would not recommend reaming the steerer to remove the extra wall thickness.
Instead, I would figure out how much stem you can get in before it binds and ensure that you always have as much of that depth used as possible.
I'd probably also say you don't want to have the stem extend out of teh steerer by more than say, 50% of the length you can insert into the steerer. Ie, if you can get 100mm of quill into the steerer then you don't want a total quill length of anything over 150mm. That make sense?
You may need to cut down the quill on the new stem but you need to get the angle right and do it neatly and you may also need to trim the bolt or replace it depending on how much of it is threaded.
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M_V
Dogtemple
Likely that the tube in the fork was only reamed out enough for the stem it came with. I’ve encountered that with seat tubes in the past.
You can check by cleaning the inside of the tube and looking for an edge or running a flat head screw driver up the inside and trying to feel for a difference. Just be aware that it will be a tiny difference, like 0.2mm sort of thing.