but we can speculate.
I would imagine the relatively high unemployment rate among young Muslim men, and the racism they face in their countries of origin might play a role.
I can perhaps draw an analogy to the drug wars here. Most young men who join drug gangs don't do so because they love drugs or have an interest in promoting drug use. They join because they see few if any other options. Gangs offer a sense of community, family even, that they can't find anywhere else. Then there's the lure of excitement, travel, being part of a "greater cause"--however twisted that cause or community might be. Of course, once you join such a group or movement the way back closes.
That's just speculation on my part. It would be interesting to see if anyone has made a serious attempt to answer your question. I expect law enforcement in places like Finland and France are working on the problem with a fair degree of intensity. You might try an on-line search to see if anyone has published anything of worth in this regard.
All that said, the percentages listed in the graphic you post are fairly infinitesimal. The highest appears to be Finland, where less than one tenth of one percent of Finnish Muslims have traveled (and some of them, as stated in the chart, might have gone for humanitarian purposes--as part of Doctors without Borders or what have you). Even this figure is probably skewed, with a small Muslim population to begin with even a few people will goose the percentages. The same is perhaps true of Ireland. And I'm surprised that Egypt and most of Muslim Africa except for Algeria and Tunesia aren't listed, not to mention Bosnia, Albania, Kosovo, and several other areas where Moslems are the majority. Maybe they don't have figures for that? Or maybe the sorts of issues of alienation and exclusion don't apply?
The one country I'd single out as an exception to almost all rules is Saudi Arabia, except from what I've read there are also a fair number of disaffected young men who see no role for themselves in a tightly controlled monarchy whose elite is closed to all but a very few. But I expect religion there plays a more predominant role--as evidenced by Bin Laden and his clique.
Finally, there's probably a sub-group who are simply sociopaths who see a "religious war" as an excuse to indulge their sadism and lust for murder. In the US, such people might grab an assault rifle and take out a school or movie theatre. In France or Belgium or Turkey they join a "jihad." If religion wasn't there, they'd find some other excuse to practice violence.
I'd be interested in reading whatever you might find by way of an answer to your question.
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