Isaiah 28:9–10 (NKJV)
9 “Whom will he teach knowledge? And whom will he make to understand the message? Those just weaned from milk? Those just drawn from the breasts? 10 For precept must be upon precept, precept upon precept, Line upon line, line upon line, Here a little, there a little.”
They are insulted. They are nation's teachers and leaders and resent being treated, as they see it, as children. These are drunken leaders who mock the word of God as infantile nonsense (sound familiar?).
"Very well, says Isaiah, since they will not listen to the LORD when he speaks to them through the simple, clear message of teh prophets, he will speak to them through the prattle of foreigners (the invading Assyrians), and the result will be not rest but ruin (11, 13)" - Webb, p120.
Isaiah 28:11–13 (NKJV)
11 For with stammering lips and another tongue He will speak to this people, 12 To whom He said, “This is the rest with which You may cause the weary to rest,” And, “This is the refreshing”; Yet they would not hear. 13 But the word of the LORD was to them, “Precept upon precept, precept upon precept, Line upon line, line upon line, Here a little, there a little,” That they might go and fall backward, and be broken And snared and caught.
Paul quotes this passage in support of his teaching in 1 Cor 14 that tongues are a sign, not for believers, but for unbelievers. The Pentecost-gift of tongues was a sign of coming judgment upon unbelieving Israel. This is why it makes sense that after the fall of Jerusalem in 70AD, the gift of tongues ceased.