Because the upasaka and the upasika still carry on the life of the householder, their thoughts are not pure and they are unable to put an end to outflows. They can only take up the four noble truths and cultivate at the stage of those with more to study. This situation is as described in a verse:

Though the peacock's body is adorned with colors,
It can't match the wild goose which can fly afar.
Though laypeople have the power of blessings and status,
They can't match the monastic whose qualities are supreme.