"first of all why can't we make a load case like L6=SUS+Win1"

Because the Code says add the stresses from the OCC event to the stresses from the SUS case. The "OCC event" (in your case) is wind, but you want the effects of wind in the operating condition - so how do you get that?

(OPE+WIN1) - (OPE) = the (perhaps non-linear) effects of wind in the OPE condition.

"why we go for SUS+WIN1"
Because you're evaluating a "primary stress failure".

The OPE case is not (typically) a stress compliance case. There is a "sticky" topic at the top of this forum that explains this.