The difference is that AMC owns The Walking Dead. AMC licensed Breaking Bad and Better Call Saul from producer Sony Pictures Television. Therefore, Sony doesn't technically have to involve AMC in any potential film; if they want to make it a theatrical release, they can. AMC wouldn't make a Walking Dead movie as a theatrical release (except maybe as a Fathom Event) because they want to keep that content on their network. But they don't own Breaking Bad, so. it's Sony's call.