The plethora of Miku designs out there is mostly thanks to the creative culture Crypton likes to cherish. It's natural that artists want to make their own designs, and sometimes those designs end up gaining attention and becoming established officially. Many Project Diva modules started off as fan designs through design contests or as CD jacket /PV designs. Different promo collaborations involving Miku often have dedicated designs too.
As for Snow Miku, it makes sense to have a design that fits the Snow Festival. Or Racing Miku, being a mascot for a racing team, is naturally given a fitting design for that purpose (eg. Race Queen). Changing these designs yearly helps keep it fresh and interesting (coughas well as being able to make new figures and goods to cash in withcough) And, especially with Snow Miku, keeps up the creative culture, since Snow Miku design contests are open for anyone.
On the topic of Sakura Miku, you have to understand how important the cherry blossom season is in Japan. Hanami is a really a big festival, so big that, for example, every spring TV channels (and other media) put out daily forecasts on when the cherry blossom starts in each part of the country. The fondness Japan has towards cherry blossom and hanami kinda explains how a cherry-themed design of Miku got so popular and why she's used so much during the season of spring. Sakura Miku started off essentially as a fanmade recolouring of Miku, but gained popularity so much it was picked up for official use too, and made into Nendoroids, figures and other merch. Since her Sakura design is tied to the springtime, it makes sense you see it around a lot now.
Sorry for the longish post, I just couldn't resist commenting. But to sum it up, different Miku designs for different purposes.