I think this is because of a fleshy appeal. The flesh wants to be in control, and this is one way, to persist in the belief we earn at least a part of our salvation.. Sadly... what occurs within Christian circles as we have all seen and witnessed is people fighting to KEEP the law... I will never understand this...
And in pushing it on others, like what God said in Galatians, they want to glory in your flesh. Interesting thing to say, that! How is making you legalistic glorying in your flesh? Because that's where legalism pushes you.
And I can already hear an objection . . . "But I'm saying holiness! Do you deny holiness?" OR some such. So to be clear, attempting to be holy, obtain holiness as a means of becoming or remaining reconciled to God is simply legalism under another name.
Of course we are to be holy! Because we are reconciled, not so that we can BE reconciled. That is always the error of legalism. Thinking that our reconciliation to God rests in ANYTHING outside of our trust in Jesus' death and resurrection.
Much love!