Even if you only have four colors or so when you cast it, it's still four Regrowth's in one spell
The fact you have to cast this only after being attacked before blockers and cant be used as a political tool to save another player is why its D-tier
They have legendaries, planeswalkers, showcase cards, and more
Three drops like Gaelicat and Tidus, Blitzball Star can rack up damage fast while evading or neutralizing blockers, and Slash of Light will be at its most effective
Snow creatures also give you the opportunity to add +1/+1 counters to your commander, which can get brutal if your opponent isnt drawing into their removal (or running enough of it in the first place)
While Toski is among the smallest creatures with indestructible, the fact that it will survive most board wipes while helping you gain card advantage cannot be overstated