2 reasons.
1) Selling bikes to people who want them for the summer, not halfway through it. Which means production runs etc need to be scheduled to finish well before summer. Even more pronounced in MX and Enduro bikes, where they’re 90% bought for racing the next season on, whatever the popular model is that year will often be sold out months before the season even starts.
2) People only buy 1 bike (usually), if there’s a shiny new Giant on sale in October, and you buy it, however good the Cannondale is that launched 2 months later you won’t buy that. So you get launches creeping forward in an effort to launch new products as soon as they can as Cannondale, Spesh, Trek, giant etc all want to launch their new bikes within weeks of the others.
Also if you sit on a new design over the winter all that R&D money (or even tooling and manufacturing if you warehouse the completed bikes) is tied up not generating a return so you can’t go on with developing next years.