Why Bible is simple.

A.  Because it is a revelation.

2 Timothy 3:16-17

 16 All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness:

17 That the man of God may be perfect, thoroughly furnished unto all good works.

2 Peter 1:21

For the prophecy came not in old time by the will of man: but holy men of God spoke as they were moved by the Holy Ghost.

Hebrews 1:1 

God, who at sundry times and in divers manners spoke in time past unto the fathers by the prophets,

B.  Because of its repeated truths. All one needs to do with the Bible is found out what God says where He says it and then believe it.

2 Corinthians 13:1

This is the third time I am coming to you. In the mouth of two or three witnesses shall every word be established.

C.  Because it is written in the most simple human terms possible. If it is understood as it is written it is simple book.

Luke 24:25

Then he said unto them, O fools, and slow of heart to believe all that the prophets have spoken:

Luke 24:45

Then opened he their understanding, that they might understand the scriptures,