Mostly around safety and pragmatics, which leads to regulatory compliance and investor preference. Invasive hardware means implanting into the brain, potentially deep brain work. This is, risky, expensive, and requires some serious expertise.

If you can get past the low signal quality of external (non-invasive) devices, then you open up a way cheaper, way safer, avenue.

Hope that helps. Can give detail if you'd like.

source: two research degrees in neurosci, and did some almost-startups in brain-computer interface hardware.