Its because pyro triggered melt is 2x damage, while pyro triggered vape is 1.5x only. The difference is overall dps since you can only melt once on the first hit, but you can vape every single hit so you get 1.5x damage for every hit instead of just 2x on the first hit.

Its sort of system to balance the reactions, those 2x damage like melt and vape has special mechanism

using pyro to trigger melt gives you 2x damage, but removes the cryo instantly
using cryo to trigger melt gives you 1.5x damage, but the pyro remains so you can melt a few times more

same thing for vape

using hydro to trigger vape gives you 2x damage, but removes the pyro instantly
using pyro to trigger vape gives you 1.5x damage, but the hydro remains so you can vape a few times more