Hugo85 is quite correct. It is regulation driven. Given that the HK license is valid for any engine size, no one in his/her right mind would get a 50cc. If you want a small engine machine for economy, compactness, and docile driving characteristics, you might as well get a 125-200 cc and avoid the disadvantages of the 50cc (underpowered entry level models). A 125cc is only a bit larger and heavier than a 50cc but a far more capable machine.
In Sweden, like in Canada, a 50cc is easy to get. No test at all. Just go. A 125cc requires a "light motorcycle" license while for the rest you need a "heavy motorcycle" license (much harder). Therefore it is logical that the 50cc and 125cc segments are well filled.