See, the majority of the Pokémon fanbase is made up of young children and the goal of a company is to make profit. The easiest way to do this legitimately is to target the core audience. Kids are easily impressed by the Fire-type because it looks "cool" to them. I can attest to this. When I played Pokémon Emerald for the first time, I chose Blaziken as my starter because I thought it looked "cool". However, some years later, when I was older and more mature, I did another playthrough of Emerald and I chose Sceptile instead. Grass has then been my favorite type ever since. I'm glad that I grew out of my "Fire is cool" phase and realized which type was actually more interesting. Basically, the reason why Grass-types get ignored by Game Freak and TPC is because Grass-types tend to have a more sophisticated design and are generally more appealing to older kids and adults, while Fire-types easily amuse underage kids without much experience in life.