Then how could they ratify anything?
The state legislatures continued to exist, even in insurrection. When they decided they wanted their representation back, they agreed to ratify the 13th, 14th and 15th amendments.
It was strictly business; you can’t call a negotiation compulsion just because the other party requires you to make concessions to finalise the deal.
The slave states wanted something back they had given away: their representation in Congress. In order to get it back they had to deal with the new amendments.
A bad deal, maybe, but that’s what happens when gamble on an appeal to arms and lose.