It's a question of demand really and maybe driven by a chicken and egg problem. Trains are slow and expensive so people prefer to fly leading to a lack of demand to support both a slow every stop train and an express point to point train. Also I imagine prices would increase on an express train compared to current trains because it will be picking up less passengers from those intermediate stations. It's tough to setup a lot of different trains because tracks are afaik one per direction in most locations and lines so you're very harshly limited on the scheduling side of things.