For the same reason you typically don't optimize for the Chinese market even though it's also huge.
There is a significant cost to compliance, even if you do it right from the very beginning (assuming it's even possible for GDPR right now), and for a lot of startups it's not worth it to spend time on that because they'll most likely be dead for unrelated reasons before they get any significant amount of GDPR clients.
It makes more sense to spend the effort on not dying instead of chasing incremental gains in markets with complicated regulatory requirements, unless dealing with those is explicitly your business proposition.
There is a significant cost to compliance, even if you do it right from the very beginning (assuming it's even possible for GDPR right now), and for a lot of startups it's not worth it to spend time on that because they'll most likely be dead for unrelated reasons before they get any significant amount of GDPR clients.
It makes more sense to spend the effort on not dying instead of chasing incremental gains in markets with complicated regulatory requirements, unless dealing with those is explicitly your business proposition.