Why? XMR Stats Ballance =0

Your balance is released in proportion to your shares only after a block is found by the pool. Since this is a very small pool, it takes time to find a block. The average time going by previous records is around a month.

Once a block is found, your balance will be updated and your shares reset. 

In larger pools, they have a lot of miners (and thus a lot of hash rate behind them), so they keep finding blocks regularly. It's not the same here. 

This is correct. Just to add, this doesn't necessarily mean that you get more reward mining on a large pool finding blocks. On a large pool, you get regular small value rewards. On a small pool, you get the occasional large value reward. Over a long period, the two should more or less equal out.