Long story, but basically SI and CNN are part of the Time Warner (TW) conglomerate.
SI is part of the "Time" division while CNN is part of the "Turner" division.
The current TW management looks at Time and Turner differently, and believe that Time serves one market while Turner serves another market.
SI will still exist at SI.com.
It's sorta like TV and newspaper being under the same umbrella, with the thinking that there is no synergy between them.