I think your pilots were banking on being able to move up a slot but didn't quite work out so you had to inadvertently sit there. Or, as mentioned, maybe the gate had to be used for another incoming flight therefore you were unable to go back. This has happened to me before at LGA. It's kinda tiring just sitting there at the corner of the airport waiting for clearance.
Its better to be on the ground wishing you were in the air than be in the air wishing you were on the ground. Fly safe!