Because in traditional Christian mythology, heaven is in the sky. This was a very common belief in the area at the time, occurring in Roman and Jewish mythology as well. It was used as a sign of approval of the gods or deification of the individual.
Similarly, hell is down.
If you’re above the sky, you’re out of bounds.
They could have just had him disappear while making an obnoxious noise, like the TARDIS. That would have avoided all of this confusion.
You could argue the Tower of Babyl story is more about hubris. Even if it wouldn't work, building it with the intention to physically enter heaven is hubris. Nobody is building and launching a Saturn V with the expectation that the astronauts are going to step off the craft in the middle of heaven.