Quicksilver Sash does not give you AD, move speed, and life steal.
TWO items built from it give more stats.
QSS only gives MR.
Youre mostly referring to 
In some cases a player may consider another AD item to be efficient/better for them.
Consider that it does have a 90 second cooldown. Many of the ultimates that you would want to cleanse have shorter cooldowns than the item, such as; Zed ult that you mentioned.
Sure getting out of jail free once every so often is handy, but some may consider that another item will benefit them somewhat more.
Merc. Scim. is a good item, but claiming that its amazing beyond reason isnt exactly fair. A case can be made for Blood Thirster being a superior defensive item due to its shield. Or even Phantom Dancer with its damage reduction. Both of those can be used more often than every 90 seconds.
Now I know people knee jerk and because I didnt wholeheartedly agree with you, it means I said the items trash and only morons buy it... but carefully re-reading my post you wont actually find that anywhere in what I wrote.
All Im saying is that some people may see the defensive options of other items as more reliable than getting out of an ult once every 90 seconds when you may still die, as the time it takes to react and hit the button to cleanse it may see you still getting blown up by damage meaning that you still died. There is no guarantee that you wont cleanse the Amu ult and then get hooked by Blitz from a bush, and Zed ulted mid air.
So Im glad its working for you, but asking why everyone dosnt buy it is like standing up in a crowded place and screaming "Why dosnt everyone agree with me and what I think!?" If youre really interested in edifying people, you generally have to show them. So make a video, create some bullet points, and present your argument instead of "This shit is awesome and youll be awesome if you use it!"
