Albert Barnes Commentary 2 Samuel 5:6

"And the king and his men went to Jerusalem against the Jebusites, the inhabitants of the land, who spake unto David, saying, Except thou take away the blind and the lame, thou shalt not come in hither; thinking, David cannot come in hither." — 2 Samuel 5:6 (ASV)

Immediately after being anointed king of Israel, David probably wished to mark his accession with an exploit that would be popular with all Israel, and especially with Saul’s tribe, Benjamin. He recognized the importance of having Jerusalem for his capital, both because it belonged as much to Benjamin as to Judah and because of its strong position.

Except thou take away the blind ... — Rather, the Jebusites spoke to David, saying, "You will not come in here, but the blind and the lame will repel you"; that is, you are so far from taking the stronghold from us that the lame and blind are sufficient to defend the place.