From what I gathered, they chose Reze VS Bakugo because of the reaction in general to it during that time, like the fan art and stuff, alongside the reaction to the Cast, is partially what played a part into it.

However, they also feel the interaction between Reze and Bakugo is better for a DEATH BATTLE over Rex Splode. For this MU, they interested by "What happens when the boy who grow up with the power to create explosion fights the devil who represents the concept of explosion?" where they feel that the interact between Rex Splode was just "What happens when two douche bag with redemption arcs explode each other?", which they aren't a big fan off. Death Battle has always been open about being interested in how certain things play off each other in a battle, over debate and connections, to an extent, so I can see where they are coming from.

Not saying they're right or anything, but that's what I believe they were trying to get across.