Why did 人艰不拆 get banned?
Chengyu (or idiom) is a cultural thing.

"Getting banned" is an administrative(political) thing.

Another thing is that your quoted message is not accurate. The original order was from 《关于规范广播电视节目用语推广普及普通话的通知》（广发【2013】96号）, which was to clarify the usage in broadcasting programmes for popularizing Putonghua.

Now why? Because Putonghua has an official standard, the official channels should keep their standard.