Why is the brain-mind (intentionally) connected to its surroundings?
That question seems very fundamental.
However, I have a feeling that what are called “biological naturalists” (or at least some of them) may see it as a “pseudo question” in that biology itself provides the answer. In other words, brains are (intentionally) connected to the environment because that was the next evolutionary step up from simply reacting – physically - to stimuli in the environment. (That can't be “intentional” - or can it?)
So here we're back to the why of intentionality and therefore the why of consciousness.
How do John Searle's “causal powers" fit into all this?
Searle's talk about causal powers refers to the fact that a certain level of complexity is what's required to bring about those causal powers which are necessary for intentionality, mind and consciousness.
Searle never never says that biological brains are the only things capable - in principle - of bringing about consciousness and intentionality (therefore semantics, in Searle-speak). He says that biological brains are the only things known which are complex enough to do so. It really is all about the biological and physical complexity of brains and therefore their causal powers.
Searle actually accuses those who accuse him of being a “dualist” of being, well, dualists (or at least some of them, depending on their overall philosophy of mind).
His basic position on this is that if computationalists or functionalists, for example, dispute the physical biology of brains and exclusively focus on syntax, computations and functions (the form/role rather than the physical embodiment), then that will surely lead to a kind of dualism. What I think he means by that is that there's a radical disjunction created between the actual physical reality of the brains and how these philosophers explain and account for intentionality, mind and consciousness.
Searle doesn't believe that only brains can give rise to minds. Searle's position is that only brains do give rise to minds. He's emphasising an empirical fact; though he's not denying the logical and metaphysical possibility that other things can bring forth minds.