Possibly because many of us recognize that the protections under GDPR are about reigning in dangerous corporate data usage, not about allowing people to curate their online image by scrubbing things they've previously made public.
There is a huge gap between "you shouldn't be able to use my data for things I did not consent to in order to commodify me" and, "I should be able to force people to get rid of any info about me that I don't want them to have, even if I made it public".
What happens when biotech enables effectively (or in practice, externally undetectable) seamless integration of artificial storage into the brain? When millions have perfect photographic memory, are they not allowed to recall the memorized account of personally acquired sensory experiences? If so, can the handful of individuals who can do so today, with no such technological enhancement, be sued for perfectly 'remembering' extensive details of someone's past public statements/actions since scrubbed from the internet? Does Alice's right to be forgotten override Bob's right to remember what she did / said?
