John Gill Commentary 2 Kings 18:12

"because they obeyed not the voice of Jehovah their God, but transgressed his covenant, even all that Moses the servant of Jehovah commanded, and would not hear it, nor do it." — 2 Kings 18:12 (ASV)

Because they obeyed not the voice of the Lord

In his law, and by his prophets.

but transgressed his covenant, and all that Moses the servant of the Lord commanded

which evils are at large insisted on in the preceding chapter as the cause of their captivity.

and would not hear them, nor do them

contrary to the agreement of their fathers at Sinai, who promised to do both, (Exodus 24:3Exodus 24:7).