The way to "get in" in the west before seems to have been difficult overall, because the "wheel of samsara" had been broken and the "stork" entity had left the area. This made it a free for all, where spirits from pretty much anywhere could get in and be born in the west. Aliens forcing themselves in, various groups summoning spirits from down on Earth to incarnate them, occultists controlling their own paths, NWOs making demon pacts to avoid hell and maintain their clique, and so on.
The maintenance deities had to compete with them and weren't able to control the fates of humans properly. This was why it was "normal" to use the walk-in method, or to not enter at conception.

The "stork" in the west is now back again, after the wheel was restored. This itself will block aliens and other meddlers from easily getting in. The population is also set to be much lower, and controlled by Gensokyo, so getting in now is harder for anyone. If there is a strong force pulling from the Earth, and the soul is willing to enter that exact location, it seems this makes it fairly easy. The maintenance deities can now make sure someone gets in if they want.

This is how it has to be done if you want in there in a new incarnation from now on.