Complex sentence: Special prominence is given throughout the work to the history of the Cross which for some reason (possibly because St. Helena, the mother of Constantine, was reputed to have been of British birth) was always exceptionally popular in England.
Split into simpler sentences: Special prominence is given throughout the work to the history of the Cross. This may be because St. Helena, the mother of Constantine, was reputed to have been of British birth and was exceptionally popular in England.