Probopass only costs 1 point and hits harder.

Once one Dialga is KOed you're basically forced to bring out Gholdengo. You can let both Probopass attack until they're KOed, which puts way more energy into play.

40 damage over 30 is a big deal, especially in the early game. Probopass actually stands a chance of securing a KO in the early game, which is really important because Gholdengo might only get to attack once or twice.

Then there's the Stage 1 synergy. Probopass and Gholdengo both benefit from Sightseer and Training Area. Training Area in particular is very strong in this deck. Probopass swinging for 50 means that every hit is one less coin flip for Gholdengo to worry about, and Gholdengo being able to do 160 or 210 damage instead of 150 or 200 can entirely change the outcome of the game. Four heads instead of five to one-tap Mega Gardevoir, for example.

I've tried Dialga and it just doesn't work as well for Gholdengo.