Question

A rocket becomes progressively easier to accelerate as it travels through outer space. Why is this so? Hint about 90% of the mass of newly launched rocket is fuel

Answer 1

because the gravity pull decreses

Answer 2

That is one answer and important to consider if the rocket is taking off from a planet, moon, or other body with sufficient mass to produce a pull of gravity. There is also another reason, as hinted at in the question:
Recall that F = ma (Force equals Mass times Acceleration).
Assuming that the same amount of thrust is coming from the rocket's engine during the entire time it is burning, the "Force" remains the same.
As the rocket travels, the fuel is burned and ejected from the rocket. The remaining fuel decreases with time and the fuel is part of the total "Mass" of the rocket. The result, the total "Mass" decreases with time.
If one rearranges the F=ma equation isolating the unknown, the equation is now:
a = F / m
Knowing that "F" remains a constant, as "m" decreases, as m decreases "a" must increase. The rate of acceleration of the rocket increases as the fuel is burned.
This reason for making acceleration easier is the same whether taking off in a gravitational filed or traveling in deep space.

It can also be meant by "the rocket becomes easier to accelerate as the fuel (mass) decreases" that if the rocket needs to be kept at a constant acceleration (maybe the astronauts can't stand more G force than what comes with the initial acceleration, the crew can "throttle back" the engine(s) as the fuel is used to maintain a constant "a". In this case, the equation F = m a now shows that as "m" decreases, and "a" stays the same, "F" will decrease - which means that the rocket engines can be "Throttled back" from the throttle setting needed as the start. Over time, less throttle (Force) is needed to achieve the same acceleration.
Hope this helped.