Probably because no matter what you pair it with, Grass is a very hard type to have 'come out on top'.
The main problem is that Grass has so many weaknesses and few resistances so pairing it makes it hard. It's naturally weak to Fire, Ice, Poison, Bug, and Flying. The only resistances are Water, Grass, and Ground. With 5 Weaknesses, there's a lot of other types that, when paired with, end up cancelling out just one of their weaknesses while adding others or magnifying a weakness. For example, Grass/Flying cancels out the Bug weakness, but then gives it a Rock & Electric weakness. Grass/Bug cancels out Poison, but makes it double weak to Fire AND Flying, plus a new weakness to Rocks.
There's really only two combinations you can use so that it's not just an easy KO every time:
Grass/Rock- (Becomes weak to Fighting & Steel while keeping the Ice weakness while every other for both types is neutralized)
Grass/Steel- (Becomes weak to Fighting and double weak to Fire, but neutralizes other weaknesses while enhancing resistances)
Basically, if you want the new Grass type starter to really see use and be the 'best starter' this Gen, you'll probably want Grass/Rock; the only other we have right now is from Gen 3 and is just a Wall, so there's plenty of room to make another good one. Grass/Steel meanwhile has a few others that it would have to compete with. Nothing stopping Gamefreak from making another Grass/Steel of course, but Grass/Rock would be the way to go.