Probably a combination of nostalgia, sunk cost fallacy, pride at having so many supported arches, a reluctance to inconvenience some hypothetical person who's s

Probably a combination of nostalgia, sunk cost fallacy, pride at having so many supported arches, a reluctance to inconvenience some hypothetical person who's still using DEC Alphas and trying to keep them on the latest kernel, and lack of motivation for someone to remove them (probably due to no one feeling that dead code is in any way a bother).