To enjoy playing this game online you need to:
1) like ski jumping
2) own a windows computer with reasonable internet
3) either enjoy using lots of time to improve/maintain your skills OR not care about losing
4) be competitivly oriented OR have friends playing the game too
5) be prepared to pay for online time
6) have a lot of free time to actually play online
(1) is limitied by nationality, time of year, possibly gender or age group, etc. In Finland, the popularity of ski jumping is decreasing because, lately, international success of Finnish jumpers has been minimal, AND ski jumping is getting less TV time and overall media attention.
(2) would seem to be rather likely but people are starting to use computers differently. Desktops are getting uncommon and non-Windows systems are gaining popularity.
(3): to become good at this game is a serious investment of time, and all the other players just keep getting better. Sometimes this game almost feels like work

. Also, the fact that this is a game purely about skill might be foreign to players who are used to the now popular "casual waste of time" kind of gaming with easy gameplay, pretty graphics and social integration. (4) to (6) should be pretty self-explanatory.
So, this basically limits the potential player group to male Polish teenagers

.
Jokes aside, there really aren't too many online players, even in the winter season:
after, say, a month of playing, you rarely see a player you haven't met before.
It seems like the most active players are skilled jumpers who take their DSJ "career" moderately to very seriously.
Which is why you mostly see them at the tournaments. Actually, there are so many tournaments now (3 weekly, 2 monthly + some occasional) that playing online besides those tournaments can feel a little excessive.
In fact, the winter season in particular can be quite exhausting and many players (myself included) are taking a little "vacation" from DSJ during the summer

.