It has been a long standing practice not to send novice mares to novice stallions so I'm thinking that ruled out The Autumn Sun. Zoustar could just be a 1 season wonder so I would think that they would think he isn't worth the risk.
Now of the leading 15 sires, on the current list, 4 are dead (including Street Cry) and 2 don't appear to be shuttling this season. I suspect that So You Think was ruled out because his progeny don't develop until late in their 3 year old season. Written Tycoon seems to get pure sprinters so I suspect that is what ruled him out. Smart Missile, whist he gets plenty of winners, doesn't get enough Group class horses so the line goes through him
Snitzel has 4 lines of Northern Dancer and with the dam of Winx having a cross of Northern Dancer they probably think that could be too many. Not A Single Doubt has 3 lines so maybe that ruled him out also. Lonhro doesn't have any but is a few years older than I Am Invincible.
Pierro horses seem to need time so that probably left Fastnet Rock and I Am Invincible - so why not go for the horse who is doing better this season.