1. Dedidueye is very popular
2. Fire is the go-to shill type because it fits the shonen archetype better than grass or water, other starter types might win the "cute" role like Piplup did but every single generation barring 5, 6 and now 9 have had the most shilled final form be fire type.
3. Most grass Pokemon have pretty...what's the word? Slow? looking designs? Either quadrupedal, a weird alternate body type (serp, owl) or are simply outright slow looking like Chesnaught and Rillaboom, which definitely impacts their popularity and just how they can be shilled. It's much easier to make a cool action scene with Charizard flying around shooting fire than it would be for Venusaur, who's claim to fame is standing still and charging a beam.
It's also worth mentioning that these days GF decides the shillmon before the generation even begins, think about how long it takes to produce merch and the anime, Sprigatito was planned from day 1 to be shilled which is why it came out the box with Protean and minmaxed stats, as well as the new anime lead's starter. Going backwards, Scorbunny was intended to be shilled--they put a soccor starter in the generation where all the gyms are giant soccer fields, same with Incineroar and Greninja, and if we really dig deep, the god of shillmons Lucario was getting pushed before Gen 3 even ended.
As for WHY it took this long for grass to get shilled for the generation, who really knows? Looking at Gato, there's really nothing about her design that hard locks her to being a grass Pokemon, she could easily have been fire or water type if they shuffled some aspects around, like a mask made of flame like Endeavor (MHA) or a cape made of water and mist illusions, but they chose grass anyways. Maybe GF thought it would stand out, maybe they focus tested the other starters and realized they'd be flops beforehand, who knows.
Gato is genuinely popular outside of shilling, so GF backed the right horse unlike in Gen 7.