I'll shut up about it after this because I know I'm obsessing, but my guess is the Ghostbusters people allowed the property to be used with the more whimsical IPs, but was not allowed to share merch with the hard R rated stuff. Universal wanted to keep a good relationship with Ghostbusters, so they stuck to the agreement and didn't put it on the house shirt to show they are following the rule to the letter. While I miss it on the house shirt lineup, I think the GB/Klowns/Monsters/Stranger Things design is a great compromise.
Also, anyone know if Horror Ink will be back this year? I really love some of the custom shirts I got from them last year.