Why do the mainstream writers all write about ancient Rome and their wars? Just look at Conn Iggulden, Simon Scarrow etc. Can't authors write about the French revolution or the American Civil war or the Vikings (not incl. Cornwell) or the Mayans or Russians or whatever. It gets tedious reading about Romans all the time.
I'll also recommend Scarrow's Generals (napoleon/wellington) series, I'll also recommend Scott Odens 2 books, Men of Bronze (egypt) and his other book (can't remember name, little help Conavar?) about Greece vs Persia.
Bernard Cornwell apparently does some good books on the American civil war (Copperhead being one of them)
For Viking tales, I'd recommend Robert Low's Oathsworn series (4 books so far) and Giles Kristians Raven series (second book just released).
I'm currently reading Vlad by CC Humphrey's, its about the man not the myth and gives a nice little view of the time.
Humphrey's also writes the Jack Absolute series which is supposedly quite good but I have tried them yet.
A coarker of a book is Tim Willocks' The Religon, a story about the defence of Malta by the Knoights of ST John from the might of the Ottoman Empire (a bit gruesome with lots of gore)
Another Waterloo book thats recommended to me is FOUR DAYS IN JUNE by Iain Gale, it's in my to be read pile and will post my thoughts on completion.
As Chopper states, Tom Holland does so good stuff, his novel on Persia being excellent and his Alezander book is also good stuff.
As to why writes cover the Roman empire so much? I would say that its probably because they were so successfull, everyone knows about the Romans so theres a ready built market. Also there alot of data about Rome, they kept alot of records, so research is readiliy available.