It might be a good idea for the previews to run on a day when a lot of other shows are dark. It may help attract more theatergoers into the show if they are not interested in seeing anything else playing that night, but are in the "I Want to Go See a Show" mood.
Phantom of the Opera is also a Monday night show.
"We like to snark around here. Sometimes we actually talk about theater...but we try not to let that get in our way." - dramamama611