I was on a commercial jet airline flight that wasn't allowed to go above 8k feet. Why?

I was on this flight earlier today (https://www.flightaware.com/live/flight/id/MXY594-1750403916-airline-1004p) and the Capt said that the ATC would only allow them to depart HPN if the airplane kept the altitude below 8k feet for the entire duration of the flight. As a result, he said, the flight would maintain 7k feet, take longer than anticipated and burn more fuel.

This makes no sense to me. Can someone explain why the ATC required this low altitude for a fully pressurized commercial jetliner?

Thanks!