There are a several reasons why.
First , bayonets have been the fighting man's weapon of last resort and has been for hundreds of years now.
Second, like many weapons, they have evolved from a single purpose weapon to a multipurpose weapon. I find it interesting to see how different cultures and countries used the basic function and used it in different ways. Need to open C-Rations or the tread in your boots or as a make shift hook in a tree to keep equipment off the wet ground or the tree itself for in the field cover.
Third, see various technologies used. Like my titanium blade used for my M9 Bayonet or the short M1 Carbine for support troops and its ease of use in area where space is a issue, like for a troop truck driver or mechanic or the unique blade design the Russians used.
Fourth, they are more affordable to collect compared to the rifles and are easier to get and ship in our post 9/11 world.
Fifth, I believe they will be used/issued less and less as time goes on. They may continue to be a option to be issued but less and less common. A well designed and made multipurpose military knife will replace what we think of as a traditional bayonet.
Bayonets are simply pieces of history that are getting long in the tooth.
Links to other Bayonet Sites
Identification of Bayonets and Markings