There's severely constrained Acela capacity for the foreseeable future.
1. The new trainsets are not yet in service; they would allow more trains, with more seats, running more often
2. Because the trainsets are delayed, some of the old Acela trains have been taken out of service for parts and to accommodate more mechanical problems
3. Construction on the Hell Gate (East River) bridge continues through 2027, reducing the number of trains that run north of NYC (e.g., Connecticut)

As mentioned above, you can look at off-hours or book in advance for much lower fares.