Thresh is too much of a "safe pick"
Remember when Maokai got nerfed because he was too much of a "safe pick" for top? What about Thresh? He's had the highest pick rate of any champion in plat+ (and the highest overall pick rate) games for a really long time now (and he's had an extremely high pick rate in LCS/Worlds). Perhaps it's because he has 1100 range with his 'q' (meanwhile Blitzcrank only has 900 range with pull). Or maybe it's because he can flay an entire team potentially. Or because he can save anyone who screws up with his lantern (and also gives them a shield). And let's not forget his ult with almost a 100% slow that can hit multiple enemies. Thresh is simply too good right now. He has the best engages among supports, and offers more safety than anyone else.