According to the site https://getaddr.bitnodes.io/, there are only 7480 full nodes (which is a very low number IMO) with most of them concentrated in the US and Europe.
For something with over 8 billion dollars in market cap. , its definetly low number of nodes.
I never leave client opened, just sync it and go away, since there is no reward of any kind for nodes , atleast that i know of.
If something should change about reward distribution, i believe there would be much more nodes.
I also think that there should be some sort of incentive for running bitcoin nodes, i think i read it somewhere that this was being proposed to the bitcoin dev team, but i cant remember where.