The more victory cards ("green") you buy, generally the less effective your deck becomes (because most victory cards don't do anything other than count for points). This means that buying VPs makes it (generally) harder to buy more VPs. The smaller your deck, the more pronounced this effect.
All of this taken together means that you end up having to target some point along the continuum between "get points quickly but fizzle out early" and "long slow burn to getting a lot of points." If you go for the "quick" strategy but the game doesn't end quickly, your opponent can take the extra time to pass you in VP, and vice versa if you choose the "slow" strategy but the game is short. And it's not just two options — like I said, there's a whole spectrum of choice.