To understand why this is we must first understand how CrowdTangle tracks pages in the system. We don't track every account across Facebook, Instagram, Twitter and Reddit. Rather, accounts must be "added" to our system. Once they're added for the first time, we'll have it forever. You can read more about that here.
The only metric that is unavailable historically from the platforms is follower count. So, CrowdTangle will only have the follower counts of pages back to the date in which the page was added to the CrowdTangle system initially.