So it looks like what happened is that the victims from the U.S. and Canada did offer testimony without physically appearing in Scottish Courts (gotta love technology) so they did contribute to the conviction in that way. What is likely is one of few things: A.) The U.S. and Canadian victims are satisfied with justice in Scotland and don't want to give traumatic testimony for a second time. While the state is the complainant and not the girls, without their testimony, it's rather difficult. B.) Frugal U.S. Prosecutors - In the U.S., the State typically tries crimes first, then the feds, and typically the States Prosecotor's offices have a much smaller bargaining and a wide berth of proprietorial discretion. It's going to cost a lot of money to get the criminal from Scotland to the Americas and that's before we have the trial... It might not be worth it to prosecute and they'll take the win in Scotland. C.) We can wait - It's not like the guy isn't going anywhere in a rush... we can fill out the paper work now, wait for about 9 years, and send it right before he gets out of Scottish Jail. The UK won't let him go out of the country with pending extradition requests for him (especially from it's greatest ally and a nation that has the same "crown" as they do). Who gets first crack at the guy between the U.S. and Canada is for them to work out, but this means that the guy can serve time in Canada and in the U.S. that is automatically consecutive if guilty... and we can make a friendly request for all that evidence the Crown collected to put the guy away. D.) It might not be a crime - Keep in mind that since the U.S. and Canada are Federal Nations, certain elements are not universally applied across the whole of the nation. In the U.S., States set the age of consent independent of the Federal Government (the one that runs out of Washington D.C.). Canada has a similar issue, just sub Provinces for States (and whatever is the Capital of Canada for D.C. I presume their parliament is in an igloo... (just joking here... can't discuss U.S. and Canadian relations without a little friendly ribbing... The Capital is Ottawa btw)).
What this means is that, if the person was of age of consent in the U.S. state at the time of the event, it's not a crime in the U.S. Alternatively, if they were consenting adults under UK or Scottish Law, but not the relevant U.S. or Canadian Jurisdiction, then the U.K. cannot honor an extradition to that jurisdiction, as it's not a crime in the U.K. This is actually quite important in extradition treaties is that the nation receiving the request has to see if it's prosecutor in their own home jurisdiction prior to giving them up (the famous Amanda Knox case, the U.S. refused to extradite Knox to Italy after her acquittal was overturned on appeal, because this was a violation of the U.S.'s ban on Double Jeopardy.).
I suspect that the likely reason (for the U.S. at least) is a combination of options B and C, as the crimes that can be charged (sending lewd material to a minor or solicitation of lewd material) is not as serious as Statutory Rape... it's a couple years in jail at best... and might not be worth the hassle. If it is, they have plenty of time to build their case before the rapist can flee the Scots.