Because you need to get them in the ground the autumn before, so they settle in. The ones they sell as bedding in boxes right now are usually hardy (like aubretia) and if you planted them in spring, you'd have to be watering them every 5 minutes, whereas over winter, they get time to put out their water seeking roots and can cope come spring.

Wasnt aware that they do? which plants are you thinking of?
unless you mean wallflowers & bellis daisies.
they are biennial, ie grow leaves in year 1 and flower in yr 2.
wallflowers are available now so they have a couple of months to get their roots down, they overwinter and then flower next year. If you bought then in March they would suffer from root disturbance or frosts would damage them.

Seaburngirl, they sell Aubretia, Arabis, Primrose/primula, Myosotis, Bellis and Sweet Williams as box bedding at this time of year

I think of bedding plants as being half-hardy annuals that you dump after flowering. These mostly sound like perennials to me.

Yes, you're right of course, Bulbaholic, but they're described as bedding because they're in those polystyrene boxes, 6 plants to a box, rather than individual plants. And the obvious one I forgot completely to mention is pansies and the smaller violas, also sold as winter bedding.

I've just potted up some plugs of primrose, polyanthus and viola which should flower from late winter until the end of spring. Just hoping the weather is right for them to grow in time.

That is a list of the winter bedding plants I get to put in the Sheringham beds at the end of October