...Is because the finisher VFX aren't a permanent feature. Even if the effect was put on a cooldown or something whenever they went off, I would buy those icons in a heartbeat if the effect was permanent, and not just a temporary eventful feature.
It makes me wish I got those special icons from URF day. 
shark music10 votesSpecial finishers should be like wards. You should be able to choose one from your collection to use at any given time. And you should be able to toggle other people's finisher visibility on or off, so the visual clutter is optional.
If they had a system like that, I would buy most, if not all, special finisher icons, at every opportunity.
Jakra1 votesYeah, I feel like it would free up some room as well. Some people might want a different icon whilst using the VFX at the same time.
disregardable2 votesSee, if they make them temporary, people will buy some every event. If they're permanent people will only buy them once.
DrCyanide1 votesIt's the trade off, because they're losing potential customers that only want to buy more permanent features, not things that will disappear after a little while. More people buy skins than skin boosters for ARAM.
I'm sure someone at Riot has a big Excel table with the math for how they think they should market it all, but I'm with the OP.
VasilisGreen1 votesThat's exactly why i won't buy them actualy. Btw, i bought the special urf finishers tho, even tho they were for the mode. I killed people all the time in urf and so the icon were worth it. I had the laser icon!!!