They're not being re-published, they've always been in print. I think what's happened is people are talking about them more so, obviously, more people are hearing about them and every few years a new edition gets released with a new cover. I have the OUP edition of Mysteries Of Udolpho from 1998 but there is a new edition out that was published in 2008 with a different cover. The same happens with The Monk, The Castle Of Otranto and even Northanger Abbey
And poppyshake, Mysteries Of Udolpho is definitely worth reading - I hope you enjoy it this time round!