Albert Barnes Commentary 1 Chronicles 21:30

"But David could not go before it to inquire of God; for he was afraid because of the sword of the angel of Jehovah." — 1 Chronicles 21:30 (ASV)

David, knowing that by sacrificing on this altar he had caused the angel to stay his hand, was afraid to move his offerings elsewhere, for fear that the angel would resume his task and the pestilence would break out again.