I was also surprised that there is not a single European country,
Last time I checked Italy was in Europe!
I can also see United Kingdom, Germany, France and Spain among which are all from Europe. Although, United Kingdom is not part of European Union which is an organization but it is still part of European countries.
About the reason Nigeria is not among, stompix is correct. But people in South West Nigeria are gambling if population is used. Many parts of the Norths are Muslims which will make gambling activities to reduce.