That list does not indicate the popularity of the different flight simulator versions.
It shows the amount of uploads for each.

Considering how long FS98, FS2000, FS2002 and FS2004 have been on the market,
of course there are more uploads for them.

Read the editors note:
Note from the editor: the FSX results are also influenced by the fact that FSX has become much more complex to develop FOR, and so there are way less add-ons being produced for it than for the more 'developer-friendly' FS2004!

FSX is the future and a lot more popular than that list indicates.