Mabye ,you need to PortForward for your router/modem.
Also ,people's won't join your server because there is much more better than your server ping, or they love to play with another map/gamemod.
Theres many reasons for this!!
First of all your server's ping is fine for all your country people!
So they can experience a good gaming!
Then the possiblity is of your lua. Perhaps your server's lua is not that good which should attract more players to join (even if the server's ping is bad)
In short its according to players to join up their favourite servers!
Often servers which is hosted with vpss are popular!
Probably servers which is hosted in germany.
Also install good modes (something new)
Its about your luck even if the lua fails
If people from his country can join then his port forwarding should be right.
The main cause probably is the ping. People usually play on servers close to their own location, because low pings = better game experience