DISH's Eastern Arc was basically a bandwidth-and-HD strategy born in the late 2000s, back when satellite TV engineering involved solving a giant orbital Tetris puzzle and pretending nobody would ever have to undo it later.
Eastern Arc officially launched in August 2008. The setup used 61.5°, 72.7°, and 77° with the new DISH 1000.4 dish. At launch, it was aimed primarily at new HD customers in selected eastern and central DMAs, including Chicago, Detroit, New York, Philadelphia, Cleveland, Green Bay, Minneapolis, Nashville and others.
TxDish
The big reason was MPEG-4. Western Arc was still carrying a lot of legacy MPEG-2 customers on 110°/119°/129°. Rather than immediately replace millions of receivers, DISH could build Eastern Arc as an all-MPEG-4 platform. That let them fit substantially more SD, HD and local programming into the available transponders. DISH specifically promoted Eastern Arc in 2008 as its first system where both SD and HD programming were transmitted in MPEG-4.
Google Groups
There was another practical reason: locals and spot beams. The eastern orbital positions gave DISH more satellite capacity and spot-beam flexibility for eastern DMAs, especially as HD locals exploded. 61.5° had already been used by DISH for HD and international programming, and DISH acquired additional capacity there when it bought the former VOOM/Rainbow satellite assets. 72.7° then became the workhorse for core programming, while 77° helped carry local markets.
CareQuest
So originally you basically had:
Western Arc: 110 / 119 / 129
Eastern Arc: 61.5 / 72.7 / 77
Programming was largely mirrored between the two arcs. DISH's own 2008 installer notice explicitly said the two systems would simulcast essentially the same programming, with some exceptions such as DishLATINO and certain international services.
TxDish
Back in 2008, Western Arc had millions of MPEG-2 boxes tying DISH's hands. Today virtually everything is MPEG-4-capable, the Hopper ecosystem is standardized, SD duplication has been massively reduced, satellite capacity has changed, and DISH has consolidated programming repeatedly. Maintaining two mostly redundant orbital architectures becomes expensive and operationally stupid once there's no technical need for it.
So Eastern Arc wasn't some weird experiment. It let DISH introduce an MPEG-4-only architecture without forcing every existing customer to replace their receiver overnight.
Ironically, nearly 20 years later they're doing the reverse transition: instead of using Eastern Arc to escape legacy Western Arc equipment, they're moving customers back to Western Arc because maintaining both architectures no longer buys them much.