Dbear said it correctly, they take up less space on a trailer. Alot of mowing outfits run 3 mowers and with a midmount you can get 3 on a 16' trailer. There is one outfit near where I live that runs only Grasshopper front mounts and I don't know how long their trailer is but they put 3 on it.
Some people say you can get a mid mount into tighter areas than a front mount, while yes it is true I have never been anywhere on a mid mount I couldn't go with my front mount. I used to mow commercially and still work for my dad doing so on occasion, I find if my Grasshopper can't get into a spot and do a nice job then dads Gravely can't either.
Storage is another issue, a midmount takes up less space.
One thing most people won't tell you, when mowing with a front mount and discharging grass into the wind it is the norm that alot of it gets blown back on to you, with a mid mount in the same situations it gets blown more behind you. Similar results with dust.
That being said I own a Grasshopper front mount and will buy another just like it if and when I get a new one in 15 or 20 years.