There are two parts of this.

One is really just geographical luck. A lot of the elements discovered by Swedes were excavated from the Ytterby mine which seem to be some kind of geological oddity, having several rare elements in relatively pure form, allowing amateur chemists and geologists to identify them.

The other part becomes a bit more speculative, but the number of amateur and professional chemists with relatively high skill in Sweden might be due to the relatively high rate of literacy and education in Sweden.

In 1640, the estates parliament required that all cities (places with city rights that elected the burghers to the burgher estate in the parliament) create a school where children would be taught how to read, write (which was considered separate from reading) and do basic arithmetics.

The 1686 church law required priests to teach children to read so that they could study the bible and Luther's catachesis and a further reform in 1737 required that children were able to read (but not write) when they were age 10. The parents were supposed to teach the children, but if they were unable, the parish bell-toller (a position that originally was a sort of church janitor and bell-ringer, but by the 1700s had become a administrative position, often held by the priest himself in smaller parishes) or an employed teacher would provide this schooling.

So, Sweden had a relatively high rate of literacy, which probably would cause a broader sample of talanted individuals being able to study further.

In 1842, a public school was instituted (it was not fully implemented until about 1850 though), where all children were required to attend school, starting from age 7-9 and for 6 years and studying reading and writing, christianity, "natural knowledge" (basic biology, chemistry and physics), mathematics, geography, cursive writing and hygienics.

Children could after the 6 years (or earlier if they had acquired the needed knowledge) continue to study at a läroverk - while the earlier education was public and free, the läroverk (roughly equilent of a modern US high school) had a cost and 1900 only about 1% of the population passed the examination of a läroverk.

Still, there were no restrictions on who could study at a läroverk or at a university, and Sweden's relatively broad middle class of self-owning peasants benefitted greatly from new farming methods, the closure of the commons and redistribution of partially owned land (skiften) in the latter half of the 1800s and some could afford to send their children to higher education.

In the end, I'd like to argue that the generation of skilled engineers, industrialists and chemists Sweden produced from about 1860 to 1920 of which some were involved in the discoveries of elements was due to this relatively high rate of literacy and education.