Probably a couple of reasons, but the first being that Rochester is not a large enough market, so Net Zero and Earthlink have chose not to offer their DSL service here.
While it is very inexpensive to offer dial-up, DSL is much more capital intensive, requiring equipment on the customers end, but also equipment to be co-located with the incumbent LEC, in this case Frontier, which can be very expensive.
If you want my opinion, you should look into cable anyway, which provides a heck of a lot more bandwidth for the money than DSL is able to.