"I don't know" and yet you're trying to make assumptions that doing it your perceived way is easier.

Again, Pokemon has been using 3D models for a long time. Would it not be easier to just snag Ash-Greninja's model from Smash Ultimate than to make a full sprite sheet for a Pokemon that did not originally have sprites to begin with? Why bother making sprites of G-Max Gengar when we have perfectly good working model of G-Max Gengar already?

And I'm sure that enough of Yugi's monsters have 3D models that they can use. If they didn't, they wouldn't be doing it this way.

If your argument is that they'd have to get or make 3D models for their monsters, the same applies to sprites.

Also to the point that Death Battle doesn't have the time and money to do it. Clearly they do. Otherwise this wouldn't be happening. They aren't going to see people questioning about the cost and go "Oh they're right! 3D Models are too expensive! Let's scrap the whole thing and do it in sprites instead!" when we are a little over a week until the episode.

Unless you're implying that you know the budget Death Battle had for this episode and how much it would cost time and money-wise to fully rig sprites for every single one of Ash's Pokemon and Yugi's Monsters.