I didn't said that population is the only criteria. But it is one of the factors that need to be considered. Look at the difference between England and New Zealand. The former has a population that is 12 times that of New Zealand. As a result, there are more cricket players at club, first-class and international level. So the player pool is much bigger. The problem with New Zealand is that they have such a small player pool, and in case any of their players get injured, then it gets really difficult to replace them.
Cricket is national sports of England, in fact cricket originated from England yet they have to wait till 12th ODI world cup (2019) to claim there first ODI title. England despite having so much good cricket infrastructure have ups and downs in there performance. In 1996, 1999 and 2003 ODI world cup they were just an average team. But this is reality that England is very strong team at the moment in all three formats due to presence of players like Ben Stokes.
New Zealand despite having small population is runner up in two ODI world cups. They are as good as any other team in the world, they just need to perform well on big days.