Because Fedclown is superior to Duck in every respect - even serve in that particular match up:
Serve - Federer wins here because Duck sucks at returning and Fed returns Duck's serve unusually well, mainly because it's all pace and shit disguise.
Forehand - LOL
Backhand - both are backhand clowns, but the geezer has more variety+better slice
movement/defense - obvious
mental - Olderer by virtue of winning so matches, so he doesn't expect to lose to Rodmug at any time
What did I say? HE'S SIMPLY BETTER doesn't cut it as there are worse players than Roddick that didn't get Roflstomped every opportunity