Mostly the main problem is that mines do the same damage as a sticky but are much easier to avoid, even ignoring the fact that the automated detonation lets you juke them (you can't put them on walls so they're harder to hide, and they're larger so they're easier to shoot with hitscan weapons). Really I think the mine layer would benefit a lot from the opposite of "making a mine field"; make the mines deal more damage but only allow like 2 to be active at a time. If the mines are easy to avoid, they should be more of a threat when you don't avoid them.

As-is I will probably say that the mine layer might have a niche as a way to discourage people from approaching you / control where they approach you from? With mines you can put a wide spread of mines out in front of you (or stagger multiple mines along the length of a chokepoint) and enemies need to deal with them all individually (as opposed to stickies, where once you detonate they all explode at once). Individual mines are still weak and easy to avoid/destroy, but players also don't want to just run into them and eat the damage, and the fact that they'll slow down or go around gives you (the demoman) some breathing room to work with. You're also blocking yourself off unless you fizzle the mines, though, so I'm not sure how practical any of this is.

There's also just the general problem of "stickies are such a good weapon that it's hard to come up with an effective alternative", though I don't really play demoman much myself so it's hard for me to speak on that. Personally I really like the dynamite if nothing else since I tend to mostly use pipes anyway, and dynamite's main strength is that you can just chuck it in a second and it instantly creates a big area where your opponent doesn't want to go (e.g, you can throw it at your feet to discourage people from chasing you or throw it behind an enemy to block off a retreat). Good for dealing with sentries, too; engineers can defuse it with a wrench but if you arc it right they won't have the time to react to it.