The short answer why: yes, just that really - I like them and want more.
The long answer:
About 25 years ago I discovered C18th literature, read Clarissa, and was hooked both on the C18th as a time and epistolary novels as a form. (I also like collections of real letters.)
Apart from simply liking them, it is a way to vicariously indulge my fantasy of having pen friends. The truth - the sad truth - is that I am the world's worst letter writer ever and wouldn't be able to carry on a correspondence.
As to why 19th century, well this year I have decided to read mostly titles from the Penguin Classics and Oxford World's Classics ranges. Apart from the Brontë sisters, Great Expectations, Jane Austen and a handful of other novels from that century, I am in the dark. It's always seemed a less than inviting period of smoke-shrouded cities and towns full of wage-slaves eking out their lives for rich industrialists. Ohh, wait a minute, and I getting mixed up with the 21st century here?
Anyway, I've decided to see what novels from the time are really like, and was curious to know if there were epistolary novels written then.
That was a bit convoluted, wasn't it? I ought to have stuck with the short answer.