More money, better league. Simple as that.
In the Deloitte Football Money League
9. FC Liverpool 203,3 England
10. FC Schalke 04 202,4 Deutschland
they are pretty close... - the figures are from 2010/11
And I think that for an international star participation in one of the European competitions is far more important than EPL or Bundesliga... especially as he is from the Netherlands where I can imagine that more people watch Bundesliga than Premier League - especially as some of the stadiums in Germany are very near to the border.
And the wage gap would really have to be extremely big if you include that he right now lives in his hometown and travels to Schalke each day - what is about an hour to drive. He has family - for him that is more than convenient... His kids can go to schools in the Netherlands, can visit Grandma and Grandpa each day...
Schalke was in the quarter finals of last years Europa League and in the semi finals of CL the year before. What is it that Liverpool can offer? Carling Cup? A run for 6 th or 7th?
I do not know if he would get weak if a European top club would approach him again (after he has failed twice) - but why change to a club without any ambitions? Liverpool certainly is not a top club anymore. It is a club with tradition - but believe me - Schalke is that, too.
I know that you all get told by Sky that the EPL is center of heaven - but last season it really did not look like it and there is good football outside of the EPL, too.