This is because of another new Modern Horizons 3 card in Ashling, Flame Dancer, which lets you keep your red mana through the end of steps and phases, which solves the biggest problem with Braid of Fire: needing to use the mana it makes during your upkeep
Let's take a look at current Near Mint prices for this card on TCG Player, Troll and Toad, and eBay
Most Used Format Non-Meta Decks Public Decks 2,786 Tournament Decks 2 Top: TCG Total Copies 5,418 Avg Copies / Deck 1.94 Main Deck 98% 5,309 copies in 2,753 decks Extra Deck 0% 0 copies in 0 decks Side Deck 2% 109 copies in 91 decks Similar Cards Cards with related names, effects, or gameplay patterns
They gain trample until end of turn
I even saw players excited to pull inexpensive cards simply because they featured a specific character