The big challenge for non-invasive BCI is that most interesting information content in the brain (e.g. speech) is encoded in high frequency firing activity, and the skull acts as a low-pass filter.
There are however a huge number of slow signals running in parallel. In total these would add up to a huge bandwidth if constrained on a single channel.
To put it another way, the scull is almost entirely resistive. And you can't make a low-pass-filter with just resistors.