Seeing Ye Are Dull of Hearing

The audience is dull of hearing, like those who will not understand the writings of Isaiah.  Milk is for those who don't understand the "word of righteousness," and "unskilful" here is the Greek apeiros, meaning "inexperienced."  Those who will get strong meat are those who "by reason of use have their senses exercised."  Like Nephi, Isaiah, and John, the author of Hebrews proposes that sensory experience of the temple is key to being able to receive and understand stronger doctrine.