I don't really understand why this thread exists. There are plenty of players of Croatian descent, but they're almost entirely North American born and who have developed entirely in North America. Imagine their was a thread for "British NHLers"? They could claim half (or more) of the league yet very few of them even identify as British and likely none of them have ever played there, much like Croatia.
Just to add some points to what others have said ...
The majority of Croatian immigration to NA occured in the 20th century (though there were Croatian sailors with Columbus!). So many people of Croatian decent are only first or second generation Canadian/American. It's silly to compare this to the Irish, Scottish, English, and French who largely immigrated centuries earlier. These people have very little personal connection to Europe. These people often consider themselves to be mixed (e.g. "I'm part English, part German, part Scottish, etc.). You'll find people of British decent, whose parents were born there, often have a stronger connection to Britain. Owen Hargreaves, for example, was the only member of his family to be born in Canada.
Another thing to consider is that it has always been common for Croatians to work abroad, especially during the communist period. You'll find large Croatian communities not only in Canada, US, SA, Australia, and New Zealand, but also in Germany, Italy, Austria, Switzerland, Hungary, and you could include Bosnia-Hercegovina, though part of it has always been considered Croatian. A number of these communities even have Croatian as an official/minority language (Burgenland, Molise, etc.). About half of all Croatians live outside of Croatia-proper. In some ways, Croatians are like Newfoundlanders.
It's not uncommon for these foreign-born Croatians to represent Croatia in athletic competition. If you look at the national soccer team roster over the years, you will see many players born in Germany, Switzerland, and Australia. I think if Croatia had a stronger hockey team (and it is improving), you would see many more Croatian-Canadians play for Croatia. For example, I would play for Croatia before Canada, but that might also be because I had bad personal experiences with Hockey Canada.
Actually, I would be surprised if half the NHL was of British decent. There's seems to be a lot of Irish, French, German, Croatian, Russan, Czech, Slovak, Swedish, Norwegian, etc.