Why it's a technique that can be performed once in a lifetime?

Since they didn't say, you can asume whatever reason. In reality, it was just used as convinient plot armor for the MC, so he doesnt get caught now, and to ensure noone can match him in the future. its a very common trope. Other than that, my guess would be because the ingredients for it are only available once in a century. Or it might be because the energy you have to collect in order to perform it, is too much for you to gather it again? Idk.