For a manufacturer is very easy to measure fuel/time with great accuracy at different throttle positions. As a plane throttle is mostly kept fixed, then you have no troubles to get the datum that suits your situation. Fuel/time for a plane is very convenient because it allows you to know how much flight-time you still have, this tells you how much you can wait for landing and how much this wait will cost. From the fuel/time is not hard to get the range of autonomy once you know the ground speed.

Fuel/distance, instead, is very sloppy for a plane as it is influenced by head-tail wind, cruising altitude and speed, aircraft weight and so on. If you need to know how much you can wait above an airport you need to first estimate the distance that you will cover and then use the sloppy fuel/distance to get an inaccurate estimation.

On the contrary, driving a car you want to know how far you can get to reach a gas station, not for how long you can drive around it, that's why fuel/distance is much better in this case.

As explained in other answers, fuel/time is better for planning, but now let's move to the cockpit!

Fuel/time is easily and accurately measured by the vehicle instrumentation and, as a plane throttle is mostly kept fixed, the value won't change much allowing you to use it to do estimations. A very convenient one is the residual flight-time: this tells you how much you can wait for landing and how much this wait will cost. From the fuel/time is not hard to get the range of autonomy once you know the ground speed.

Fuel/distance, instead, will make your life much harder if you need to know how much you can wait above an airport!

On the contrary, driving a car you want to know how far you can get to reach a gas station, not for how long you can drive around it, that's why fuel/distance is much better in this case.