Growing mushrooms outdoors is generally easier for most species. When growing indoors, the artifical environment is being designed to really allow any fungal growth to have the perfect growing conditions in a sterile growing medium; so whatever gets a foothold first will colonize the medium. So we try to introduce the mycelial growth in a way that gives it a head start over other undesirable spores in the environment.

Outdoors, this balance is maintained perfectly by nature, so the mycelium doesn't have to fight the environment so much, and there is a lot more space for the mycelium to expand until it finds perfect growing conditions. In a tub, there isn't much room for the mycelium to "fight" before the growing medium is compromised.