Why did God make so much of the Bible hard?

William Whitaker suggests 9 reasons though he says "other causes more besides these might be adduced":

(1) So that we pray for help in understanding it

(2) To excite our diligence in studying if

(3) To keep our interest

(4) That we might appreciate the great truths obtained by great labour

(5) To keep us humble

(6) So that God’s word is not exposed to dogs and swine but only to pure and holy minds who appreciate its value

(7) So that we give time to them and are not entirely taken up with external things and our daily occupations

(8) To accustom us to internal purity and sanctity of thought and feeling so that the profane lose their trouble and oil

(9) Because God intended to honour the Scriptures and the ministry, some being teachers and some disciples

(Disputation on Holy Scripture, p365f)