It's about speed. Getting into space is relatively easy (it does take a lot more propellant than flying an airplane, but it's understandable since spaceships tend to fly a lot higher than airplanes). Staying there means you have to be flying so fast that you fall continuously around Earth instead of into it. Even the fastest airplanes in the world can't get that fast on their own (at least not yet). More speed requires more fuel (and since that fuel makes you heavier, you need even more fuel to counteract the added weight - that's the rocket equation).

Even more intuitively: it's about distance. Flying from Los Angeles to Salt Lake City requires less fuel than flying from Los Angeles to New York City. Flying from Los Angeles all the way around the world and landing again in Los Angeles takes even more fuel.