In my limited experience, driving across the UK on FM is a pain due to regional differences. A couple of hours driving and I'll need to retune a few times, and my saved stations won't be applicable. DAB solves that minor issue at least.

I don't know the details of the spectrum usage, but there are more niche nationwide stations on DAB (Planet Rock is hardly niche anymore tbh, and is great). So maybe it's more accurate to say the FM spectrum isn't cluttered for barrier to entry reasons.