Having just come back from Europe a week ago (and having endured many a grey European Winter during my youth), I have a very clear answer to your question:
Because it’s frigging awesome in Summer, that’s why!
Granted, German Christmas markets and some Gluhwein are very special but after my Aussie partner experienced what I call “the real Winter” (i.e. grey snow mud in the streets of Berlin, grumpy faces everywhere, drizzling rain-snow mix with iced up motorways/ train tracks…) for the first time, we’ve both decided that my mum and dad will have to live without us over Christmas from now on. Give me tacky tourist Christmas at Bondi beach and a couple of barbies on the balcony over this any day!
The side seasons make lots of sense and for quite some time we more or less regularly went around September/October to include a couple of Mass at Octoberfest in Munich (if anyone wants tips on how to accomplish this best without queuing up together with other tourists outside the beer tents- DM me, we’re quite the Octoberfest experts these days). Last year though it got very very cold in mid September already, I fell ill with pneumonia and then ended up in hospital. So Summer just felt… More attractive this year.
And what a glorious Summer it was! The sun stays up much longer in Central Europe than it does down here and the fact that you can just sit outside till the late night with drinks in hand us just amazing. We enjoyed Germany, France and Hungary this time around and while it was hot, we very much enjoyed the atmosphere and the action till late at night. For us specifically, it’s also in Summer when the European gay prides are on which alone is a reason to chose this season. But for others it might be other events that are only on in Summer and weather it’s concerts or festivals or whatever it might be that takes your fancy- all those are just so much more special in the Summer time.
Just my 5 cents on the matter. I quite enjoy the Christmas holidays down here in the sun and then escaping the Aussie Winter over June or July. Oh, and mid July is also my birthday so that’s also extra special with old friends and family in Europe