The major reason the production stopped was the movie's ballooning budget, which some reports have clocking in at $250 million. Producer Jerry Bruckheimer managed to get the budget down to $232 million, but Disney wanted it at $200 million even, and wouldn't go above that number.
Another reason the shoot, which was to take place later this fall, has been put on indefinite hold is because of Walt Disney's ballooning live action slate, which includes the potentially risky John Carter and Sam Raimi's Oz: The Great and Powerful.
Even though Lone Ranger was in pre-production, meaning money has been spent on the project, and that Johnny Depp will get paid his full salary whether or not the film gets made, it was the easiest film to pull back on.
Lone Ranger could move to another studio, or it may still get made by Walt Disney, but a long time into the future.