Fair enough. And a bit snarky from me, perhaps, so apologies if that's gone down wrong.
But seriously - and I hope there's no reason to take offense at this, as I'm not implying anything beyond the simple point - a Scottish brogue* can be very difficult for some non-Scot Anglophones to understand. There's no reason to suggest that this is any more the 'fault' (the wrong word, probably) of the speaker than the listener. And, hey, maybe it's just me - if I'm wrong, it won't be the first time. But it'd be a fairly straightforward answer to your query of 'why?'.
Hopefully that's come across ok. No offense intended.
*not that Barry Bannan's speech strikes me as such.