The Korvold combo uses
Food Chain with
Squee, the Immortal to make infinite mana. Then with Squee on the battlefield, cast Korvold and sacrifice Squee to draw a card. Recast Squee from graveyard.
Exile Korvold to the command zone using
Food Chain, and then the loop repeats. This way, you draw your entire deck to look for the outlet.
With the Prossh combo, you need to dedicate a bunch of card slots to finishers. These cards are for the most part useless outside of the combo turn, which is why they are often refered to as dead cards. In your deck, you've dedicated 9 slots to finishers, which is about 8 dead cards (
Goblin Bombardment is the only one with good utility outside of the combo turn). Because you can draw your entire deck with Korvold, you can effectively eliminate all of the dead cards from this list, freeing up more space for combo cards, including a back-up combo that doesn't work with Prossh.
You may be worried about not being able to find Squee, but it's actually really easy to get him in
exile, thanks to
Demonic Consultation,
Tainted Pact, and
Plunge Into Darkness. These cards can easily get you
Food Chain, and
exile Squee in the process.
The backup combo uses
Dockside Extortionist and
Temur Sabertooth to generate infinite mana. Cast Korvold, and then keep sacrificing the treasure tokens to draw your deck and win.
Furthermore, Korvold offers incredible value that Prossh just doesn't provide. Even using a fetch land will draw you a card with Korvold, making him immensely useful outside of just the combo.
In summary, Korvold uses less dead cards, has a back-up combo, and provides an easily accessible source of card advantage. This makes him an all-around more consistent and stronger combo deck than Prossh, who provides none of these things.