Contrary to the premise of this question, the theater did not decline during the Victorian period. In fact, it flourished. In England, more plays were publicly performed during the nineteenth century than during any previous period. Queen Victoria herself was a patron of the performing arts, and she frequently encouraged drama during her reign.
Additionally, during the nineteenth century, public transportation improved, with the use of rail and trolleys. This allowed more theatergoers from surrounding towns to travel to London and other urban centers to attend the theater. Streets became safer in the evenings, which allowed shows to run longer and later so as not to conflict with the workday. Improvements in lighting also allowed plays to run into the evening. This opened up more opportunities for the working classes to attend the theater. It also removed the stigma of theater as a place for unemployed vagrants.
All these factors led to an increase, rather than a decline, in drama during the Victorian era. With more ticket-buyers, more theaters were opened, and many acting companies established. By end of Victoria's reign, London was home to sixty-one active theaters.