I think the window for the picks was really short (like 3 days or something).
I'm not sure why Riot locked the games that haven't been played yet. Especially for the second half of the bracket which really doesn't have anything to do with top half until the final game. I mean if you haven't gotten a perfect game so far I don't know why they wouldn't let you make later picks to get the icon which is adorable.
And the window to do this was Monday - Wednesday so if you are in business or actually have to work a normal shift it is easy to miss out on. I mean they could have sent out reminders to the players who hadn't selected anything yet, It really wasn't highlighted in the news section of their website for very long. They tend to spam that top articles all at once so unless you saw the reminder on the day it was posted, odds are you would miss it on their site.
Not only that but the drop off of players who made picks from the first round to the second round is massive. 2.25 Million made group picks, but only 1.4 made knockout picks? That is almost 1 million players who probably haven't realized they missed on on playing.
Hopefully Riot helps you and all of the other players who really love Poros. (I'm not sure why they would make something so freaking adorable the icon for an event that has nothing to do with Poros anyway)