Why can't I mine something which resembles Crimtane ore?

What you've got there is Crimstone, not Crimtane. You'll need a Nightmare or Deathbringer pick to mine that.

You could also use explosives, or Purification Powder will turn it into normal stone.

Crimtane Ore, same as Demonite, can be mined with gold/platinum pickaxes. The Crimstone (entirely different from the ore) itself, requires a Nightmare pickaxe (demonite) or the Deathbringer Pickaxe, which is the one made from Crimtane ore.