I know from IRC, this forum and some server talks that Quakeworld servers are vastly emptier during summer time. Can someone provide me with an explanation why it is so?

Because qw isnt ppls number one priority anymore.
When it is warm etc ppl dont stay at home as much as when it rains and it is Cold.

Thanks. That seems like an obvious reason, but what puzzles me is why other games don't seem to experience the same phenomenon.

It's just because the other games have quite a bigger community

also other games are mainly played by teenagers, 13 years old, who still prioritize games over women... we QWers are mostly 20 years old +
(up to 45 years old +) kkkk

Historically (10-13 years ago or so) the main reason IMO was because a large proportion of the player base were Scandinavian students who would leave their student accommodation complete with 99999999gbit connections over the summer months and not be that keen on playing on dodgy connections they had at home. Also in Scandinavia the impression I get is that summer is more of a big deal in terms of getting outside, socialising and so forth (possibly because it is so cold/dark in the winter months so they want to take advantage of summer), so again, less people sat at home in front of their PC. There may be some other factors relating to the summer break from education as well such as people working/travelling more etc.

Nowadays a smaller proportion of the player base are students but I imagine summer still holds the appeal for being outdoors, plus it has been this way for so long now that it is just expected that the competitive scene hibernates for a while. It is kind of like a 'self-fulfilling prophecy' in that everyone believes that everyone else will be idle over summer so they have no motivation to try and play.

Personally before I started working the QW summer break used to annoy me a bit because to me it seemed the perfect time for playing as I didn't have to get up early for lectures, exams etc, I was quite happy playing all night.