It is not just the milder weather alone that makes BC attractive to many, it is also the wide variety of landscapes and climate zones within very short relative distances that one can explore.
Every province has beauty and variety (not going to debate that!) but BC is the only province that has rain forest (home to some of the world's largest trees), world class ski resorts on mountains up to 2 / 3 km tall (there are taller mountains but no ski resorts are on the highest peaks), and sage brush / cactus semi deserts nestled in deep canyons home to a thriving wine industry with average summer highs over 30 degrees C in places all within half a days drive (or less) from each other.
Then there are other aspects the bring people as well, for me the variety of landscapes and biogeoclimate zones will always be what brings me back to BC.
Ad on that note, I think I just discovered my new favorite climate station in Canada (Oak Bay Victoria), and once again affirms my belief that Victoria truly is Canada's one climate outlier.
This is accompanied with only 638 mm of rain annually (around half of Vancouver) and almost 2200 annual hours of sunshine. July averages only 10.5 mm of rain and 328 hours of sunshine!
So if you are going to do the move to BC, you may as well make it Victoria IMO, there are still relatively decent prices for retirement areas in the burbs (and relatively near downtown).