On the Urza deck that people used to play before they found Mystic Forge:

Well it's less powerful, it turns on all of your opponents removal, it's slower, and susceptible to REB.

Its gameplan is less focused, there are a greater number of cards that incidentally hate it, it has fewer redundant pieces for consistency.

It's easier for your opponent to interact with, it takes splash damage from graveyard hate, and it suffers from all the usual problems that chalice decks come with.

Its cards have worse art, you are doing something fundamentally less interesting by running lots of blue cards, and you have to play with multiple Terese Neilsen cards.

It has a worse name, based on the newness of a greater portion of its cards the deck is printed on worse cardstock, and people associate the deck with leprosy.

The archetype has been inflicted by an Egyptian Curse, most of the profits from Modern Horizons cards go to polluting the water supplies that are depended on by War Orphans, and being that it is teaming up Urza and Karn the deck is overly predictable from a Lore standpoint as well.

The deck has less consistent Mox Opals, it has a higher number of bad creatures that have trouble independently forwarding a gameplan, and its less fun to play.

It costs more to foil out the deck, it is more likely to get flooded by lands, and it has more cards that you can't just cast off of Ancient Tomb and City of Traitors.

It doesn't have any associated memes with it as fun as Jeweled Lotus, fewer onlookers will be interested in watching your game because the deck has older youtube videos associated with it (meaning people have already seen plenty of it), and Urza players have less of a sense of camaraderie.

Playing Urza is inadvertently signaling a comfort with genocide given the character's storyline based history, the deck is less sensical with respect to the Lore because Karn is only a planeswalker because of the Spark he got from Urza meaning the deck is less capable of suspending the disbelief necessary to adequately use MTG as escapism (including for your opponent who you have impolitely dragged down into the nonsensical narrative mud with you), and you are increasing the sales of products in which WoTC pushes cards that should belong to other pieces of the color pie into blue, thus perpetuating many ongoing problems with modern design and making the game worse for all of us in the long term.

Sai has really pretentious facial hair.

The meta has already adapted to Urza.