There are quite a few possible reasons for this:

• Cost. It's cheaper to cut a screw thread on the end of a lens than it would be to retool to make a bayonet mount.

• Sealing. Some Canon L series lenses are only weather sealed when a screw on filter is installed. It's easier to make a screw thread watertight, a bayonet would require very tight tolerances or a rubber seal.

• Compatibility. This is a biggie. Almost all camera manufacturers have their own line of lenses, thus switching to a propriety bayonet lens mount can reap commercial rewards (in addition to providing a better mechanism for mounting). In contrast lens manufactures don't always have popular filter product ranges, it would be an unpopular move to switch to a proprietary filter mount, as initially there would be very few filters available.

  Lens hoods tend to be made by lens manufacturers and often only go on only one lens. Filters are near universal. There's usually an outcry when manufacturers update a lens and use a different filter size, imagine what would happen if an update suddenly rendered all your filters obsolete. Even if a bayonet to screw adaptor was offered, it wouldn't be feasible for all lenses due to vignetting issues.

• Stacking. Unlike lenses, filters are often stacked, so all the increased costs of a bayonet mount on the lens and compatibility issues also apply to the filters themselves.

Except in some extreme cases (Hasselblad can do what they like, once you've laid down $80,000 for a camera body, filters are loose change) there are standards which will never change, even when a technically superior solution is offered. Look at the standard accessory shoe. Despite being almost a century old Sony stand alone in offering a different design. Despite the Sony iISO shoe being superior in many ways no other manufacturer has followed suit and their decision is largely unpopular with Sony users.