The majority of these flights are to serve inclusive holidays, so it is really just down to which hotels have done deals with tour operators from certain countries, and the flights will follow. Yes, German operators dominate in the minor Canary Islands, UK ones dominate at Sanford, etc. From Finland there is more traffic to The Canaries than to mainland Spain, despite the additional distance. Ireland in contrast is the reverse, favouring Malaga over The Canaries.
Hotels (or groupings of them) will form associations with favoured clients over the years, as in all business areas. In addition they will try to have staff that speak the language of the incoming customers, and they cannot do this for all languages.
There are also cultural traditions. Malta is predominately a UK destination, although they are now building up from Eastern Europe and Russia. Ireland has strong links with Italy, and Germany with Turkish resorts.