Why is there no express bus or train from Montreal to New York?

Amtrak has a train called the Adirondack. It goes from New York's Penn Station to Montreal's Central Station (Gare Central). It is NOT an express, as it stops in about twenty little towns through New York state. It costs between $100-$150 depending on what dates you travel. Because of all the stops, it does take about ten (10) hours to do the trip, but considering driving between Montreal and NYC takes 6-7 hrs, that's not much more time and you can relax, not having to drive it.

If you needed to get their quicker, you could fly. But it costs three times as much, and while the plane ride is only 90 minutes, you would still need to factor in the time you must spend getting through airport security and getting your luggage afterwards. Expect at least 2-3 hrs extra for that.

Geryhound and Trailways bus lines both offer service between Montreal and New York. It's slightly faster than the train because they don't make as many little stops. But I'm not sure it's as comfortable. And costs about the same ($145 for a return ticket).

There isn't enough demand for one. There already is a bus that only stops for US Customs and in Albany and the US Customs stop is mandatory for any cross border trip. Anyone who wants a fast trip can fly.