Few thoughts off the top of my head:

a) Language barrier — Spanish is only the fifth most spoken language in the EU, with only 15% speaking it as a first or second language (around half of whom are Spanish citizens);

b) Lower salaries on offer (as others have noted) as opposed to working in the UK, Germany, or France;

b) Given the economic situation in the country, and high levels of unemployment, Spain isn't anywhere near as attractive to skilled migrants as the other leading EU economies.