The reason Federer has never been on the cover of SI has nothing to do with Federer & everything to do with the fact that tennis is a completely irrelevant sport in the US since the 1980s.
Tennis players(not just the American ones) were on the cover all the time in the 80s, & virtually disappeared from covers since the early 90s
Sampras, a 7 time Wimbledon & 5 time US Open champion was on the cover of SI only 3 times in his entire career(& wasn't on the cover when he broke the slam record in 2000)
Agassi, one of the most popular champions, was only on the cover 3 times.
Courier, Chang, Roddick, Hewitt, Safin, Kuerten, Davenport, Hingis have never been on the cover of SI.
I think Agassi in 1999 was the last male player to get an SI cover. Since then Venus, Serena, Kournikova, & Sharapova are the tennis players to get a cover.
Since Sampras was so uninteresting to American sports fans, I wouldn't count on Federer ever receiving an SI cover, even if he wins the calendar Grand Slam(which would coincide with week one of the NFL, so it would be kinda stupid to put him on the cover)
don't know much about Schumacher, but just because Tiger won six tournaments in a row, big deal. Federer has won 4 tournaments in a row more than once I believe, but would that win him the Laureus? I don't think so. 3 GS's in one year and a (so far) 72-5 record is the big issue. Tiger won, what, two? I don't see it as equal to Federer's credentials this year.
You don't know much about Tiger either, apparently. Compare Tiger relative to golfers accomplishments & his streaks/records are historic. No one has ever had this sort of domination in golf. No one has won 6 in a row in golf since the 40s. Many have won 6 in a row in tennis in the open era.
Not the same with Federer & tennis(& tennis only became a pro sport in 1968, while golf has always had a long history of accurate records) Laver, Budge, McEnroe, Connors so many others have had streaks/years similar to Federer.