Beta testers focus on tangible actionable feedback, like: "game crash in x situation" or "y character's arc could use restructuring". Whereas early access feedback is more like "these parts just aren't fun" or "man, i, j, k, l, & q would be so cool!", which isn't nearly as actionable, but arguably more useful to the end product. Though good luck bug hunting with "game bork pls fix".

At the same time, beta testing isn't fun, it's work. Trying every combination of buttons to find edge cases can be mind numbing, and playing the same thing 400 times in one day can drain any appreciation of the game as a whole. Early access players can actually play the game, and leave if it's not fun. The majority won't even leave feedback anyway.

Also, beta testers often need to sign NDAs and could loose their career by breaking them, while early access is straight up letting the cat out of the bag. Depending on how replayable or spoilerable the game is, that might be undesirable.