I'm not too into it. I would assume the economic benefit of hosting the tournament to a region or single country would outweigh the costs, something a potential host would consider prior to making a bid. I think having 45 countries hosting it would create a mess for scheduling, could create bias's in some teams randomly getting home field advantage, while other games drawing few fans - 2 small countries playing in an equally and uninterested small countries stadium. The idea of small countries building a "national stadium" for the opportunity to host 1 or 2 possible matches without knowing the quality of sides playing appears to be even more of a risk than preparing for the entire tournament.
Article is from Dec.6th 2012 . There are numerous cities hosting though, they are: Amsterdam, Baku (Azerbaijan), Bilbao, Bucharest, Budapest, Copenhagen, Dublin, Glasgow, London, Munich, Rome and St.Petersburg.