The main reason was described by R.A. Salvatore in interviews. He describes how the complete overhaul and moving the setting by 100 years would ruin his characters; a 100-year old human is no longer a good fighter. He even states that after the decision was announced in a private meeting between the FR authors and WotC, Ed Greenwood was completely devastated. They decided to immediately work on a plan to bring FR back to how it used to be.
You can watch a Sword & Laser interview here. The relevant part starts around 11:50. Quoting some parts:
Fourth edition, it was a shock. ... I am sitting there thinking, well, 140-year-old humans don't fight very well. ... I actually wrote a long letter to Wizards, ..., and said please don't do this. There are other ways to accomplish what you want. ...
When Ed Greenwood and I walked out of that meeting back in 2006, when we were told about the reboot for fourth edition, Ed looked at me and I thought he was going to start crying. I mean, these were his Realms, that had been taken away from him essentially by this big change. And he said to me what are we going to do? And I said, we are going to be smarter than them. We are going to think long term. ... We started planning back in 2006 how we were going to fix it for them.