Because Blackpool have sold *some" tickets in all the blocks they had tickets for, which means that the spare seats in those blocks can't now be sold to West Ham, where there is still high demand for tickets.
Blackpool have been to Wembley a few times in recent years, as well, which tends to lower demand.
FWIW, last year Swansea and Reading sold out every single possible ticket they could have, including the Club Wembley returns.
Albion would sell out with no problems - if only because of the...