The line doesn't need to move quicker, and Ukraine moving slowly and shoring up every meter they take is going to work out better long term. They aren't on a march to invade their lost territory, they are on a march to the Azov sea to cut Russia's only remaining land access to Crimea.

They are building a wall of men and weapons and once that reaches Azov they only have to defend it - and Ukraine are very good at defence.

Then the only way Russia has to get anything to Crimea is the Kerch bridge which is almost inevitably going to be destroyed.