Why are parties of 4 constantly disabled in Ranked Flex?

Mainly because majority of solo players go for solo queue. So teams of 4 would have huge waiting times in the queue. They are giving out rewards to players that rank up in both queues so the issue won't be as big as they are planing to enable 4 man queues later in the season.

So they wont be toxic to the solo player that gets paired up with them, besides other things.

They said they would disable 4s until they had enough data to determine if allowing them would break queue times based on the number of solo players in flex.

I play solo flex all the time. :)

well who would duo in flex instead of solo/duo?

running a party of 3 in flex you either get a duo or 2 solos, both of which can be queued in solo/duo. So wouldnt it be easier to wait for 1 solo than 2? [sg-lux-2]

I believe it is to discourage Solo Players from using Flex. If you allow groups of four, you are basically stating you need Solo players for Flex to work. Flex Queue is not meant to compete with Solo Queue over Solo players.

Riot realizes that no one wants to be a solo with a premade group of 4. Somehow they haven't come to that conclusion for normal drafts, even though hundreds of people have posted asking for a "solo only" option in draft.