There are really very few companies in the commercial GTO launch business, and the market has changed greatly in the last decade.
International Launch Services (ILS) intially formed with Lockheed and several Russian and Ukranian companies to sell the Atlas V and Proton. However, Lockheed quickly sold it's stake to another company, and took its Atlas V with it. ILS is popular as they can do a lot of launches in short order and are reasonably priced. They have had several hell known failues, but the fact is all systems have them. You just hear more about the Proton ones because they do so many commercial launches, including DirecTV and Dish, and have such a huge chunk of the commercial market share.
The United Launch Alliance (ULA) is a joint venture between Boeing and Lockheed and market the Atlas V, Delta II, and Delta IV. This is where Lockheed went after pulling out of ILS. They mainly do government jobs, including weather satellites, science missions, and spy satellites They government likes them as they don't want to send their top secret stuff out to the US. As such, they command a premium most commercial launchers don't care to pay. Nonetheless, they have done few commercial launches.
Arianespace markets the European Space Agency's Ariane-5. It has costs comparable the ULA, but has the advantage of being able to launch 2 medium sized sats at a time, meaning each customer can pay half. However, the latest Dish and DirecTV sats have been a little too big to split the cost.
Sealaunch was out there, but went bankrupt, and have only had 3 launches since mid 2009. They lose a somewhat weaker rocket (a Ukrainian Zenit), but partially made up for it by launching from the equator. But with so few launches, not a lot of space to get on their schedule as they try to get back on their feet.
Several other governments have launchers, but I don't know if too many do commercial launches. Charlie's first sat was sent up by the Chinese, but it caused controversy over sending US technology over there. But essentially, there are only 4 options in the game.
Here is the history of Dish launches
Echostar 1: Chinese Long March 2E, 12/28/1995
Echostar 2: Ariane 4, 9/10/1996
Echostar 3: Atlas 2, 10/5/1997
Echostar 4: Proton-K, 5/7/1998 (Note: satellite failed)
Echostar 5: Atlas 2, 9/29/1999
Echostar 6: Atlas 2, 7/14/2000
Echostar 7: Atlas 3, 2/21/2002
Echostar 8: Proton-K, 8/22/2002
Echostar 12: Atlas 5, 7/17/2003 (Note: originally launched for Voom as Rainbow 1)
Echostar 9: Zenit 3, 8/8/2003
Echostar 10: Zenit 3, 2/15/2006
Anik F3: Proton M, 4/9/2007
AMC-14: Proton M, 3/14/2008 (Note: failed launch)
Echostar 11: Zenit 3, 7/16/2008
Ciel-2: Proton M, 12/10/2008
Nimiq-5: Proton M, 9/17/2009
Echostar 14: Proton M, 3/20/2010
Echostar 15: Proton M, 7/10/2010
Quetzsat-1: Proton M, 9/29/2011
Echostar 17: Ariane 5, 7/5/2012 (Note: built for Hughesnet HSI as Jupiter 1)
Echostar 16: Proton M, 11/20/2012
So, it is only recently Dish has been on the Proton kick. They have long used several launchers, and made ample use of SeaLaunch when they were around. But there have been several changes since. Satellites built before Echostar 11 were in the 3000-5000kg range. After, they are in the 5000-6500kg range. That has somewhat trimmed the options. That has excluded lighter ULA options and easily pairing with an Arianespace option. With Sea Launch so limited and on the cusp of their capabilities anyway, ILS Protons are simply the best option for Dish of late.
By my count, there have been
Proton: 10
Atlas: 5
Sea Launch: 3
Arianespace: 2
China: 1