Why does a sport certificate have no distance restriction while recreational does?
Because a sport pilot certificate requires the applicant to undergo at least three hours of cross country flight training whereas a recreational pilot certificate does not have that requirement.

I always got the impression that a recreational pilot certificate was essentially for people who had soloed and did not want to accomplish any other aviation goals save flying around their local airport.  Instruction is restricted to pre-solo aeronautical knowledge and skills.  It’s pretty limited in capability compared with a private pilot certificate, which explains why there are less than 600 recreational pilots registered by the FAA.

And one can expand a recreational pilot certificate to permit PIC on cross country flights over 50 nm, but this will require additional training and logbook endorsements by an instructor