While Puppetmaster can make sense from the perspective of Mammets being more focused in the story over time, I think Beastmaster just makes more sense in terms of how much work they want to put in a limited joband there was a whole beastmaster thing in Bozja so it's not entirely ignored either). They already have a ton of sprites and animations for beasts. For mammets well there's a few models from the few dungeons like knight/caster and the twin dolls and that kind of stuff but feels like they'd have to just make a lot more assets. If they want puppets to be actually customizable, they'd need to have a system for that which I don't know how much they can do with "summons" wearing "gear".
I think beasts work better from a collecting point of view too, with a system where you have a "stable" of pets that you can swap on the fly with a few skills each so there's incentive to collect a lot of different things to work together, rather than building a puppet with maybe a few presets. And there's only so many pieces you can have in a puppet to collect, it's a bit hard to add more without making it "the same thing but a different color", I think? Unless they go with the puppet being assigned a class in which case puppets can be upgraded every expansion with the new classes being added and such, but still leaves the question of how different a SAM puppet would be from a NIN puppet, besides how it looks. In XI a lot of puppet pieces are just better stats and few actually change the puppet properties.
So overall for the gameplay aspect of a limited job that would likely be focused on collecting a lot of stuff like BLU is since you can't have them in standard content, Beastmaster just looks like a better choice. You can collect the beasts, maybe they can be trained to unlock more skills, you can have them seamlessly integrate into combat system similar to how SMN works by pressing button and a beast comes out for a skill or by making it "stance" based where you switch to a beast like a stance and get access to specific skills there until you switch to another and such. There's less reliance on pet AI and stuff if you do it that way, while PUP would be entirely pet centric and pets in XIV are... not great. If you've played SMN before the rework you know how finnicky some stuff is.
