Adsense! Networks are not worth giving away a cut of your income to, especially when you are under 20,000 subs. Say your YouTube income is like a pie...
YouTube takes 45% of that pie already. Now you join a network which has a 60/40 rev share. This means that after YouTube leaves you 55%, you then give 40% of that to the network. So out of your original, you now have less than half? I went there once, and you won't catch me there again unless YouTube makes me a millionaire, and maybe not even then!