A lot in my opinion is to do with the culture. Having lived in Croatia for 17 years, there are some stark differences to Scotland among the youth.
Sports that are mainly popular (successful) in Croatia are Football, Basketball, Handball, Water Polo. Young kids follow these sports and aspire to play at a national level.
Croatia does not have a pub culture like we do in Scotland. Yes, there are cafes and bars (you can drink alcohol anytime they are open), but your young folk will generally drink coffee, soft drinks mainly, unless it is a social occasion when they will then go on a bender
Average salary in Croatia is probably around 4,500-5,000 kuna a month (around £500). Making it in sport (like Modric, for example) allows the family a lifedtyle they could only dream off, so the kids are encouraged to excel.
There is a much healthier diet in Croatia (only recently has McDonalds, KFC, entered the market. Might be interested to see what the next generation health will be like!
Until recently kids did not have access to the 'toys' of the likes of Play Station, etc, so like us Scots in the 60's and 70's when we were out kicking a ball around or similar, thats what inspired to an extent the successful players they brought through the ranks.
But in reality what has made them head and shoulders above us is that they drink rakija at 7am with their coffee and cigarettes