It isn't though. Player counts for other RTS games that aren't imploding (DoW3's 25 fold collapse in a similar amount of time is gigantic compared to the 5 fold decline in SDN44 numbers) or dead (most recent RTS games, all of which have peak player count of 150) rose with the onset of summer.
In fact increased player availability in summer is such a well known phenomenon that players in eve online planned major campaigns around it.
You may be right, but that a number of other games have an increased number of players during holidays is no proof that it has to be that way in general or for SD44 in particular.
That only works if the games you compare SD44 to have a similarly structured player base and games take a similar amount of time (among others, I guess). For instance, if your player base is mostly made up of teenagers still in school, summer holidays may have a totally different impact than if you have, say, pensioners who can basically play whenever they want but use the good weather to work in the garden or something. Or, if your job only gives you an hour a day for gaming, you pick SD44 because you can just fit one match in, whereas during the holidays you can finally take the time for that epic grand strategy thing you always wanted to play but just couldn't get into with only one hour a day...