The news isn’t coming as a big surprise, for a few reasons.
First, BNDeNA has released pretty solid games such as Gundam Kingdom and social 3D shooter Macross SP Cross Deculture - however, it never landed a mega hit (both titles will be closed down over the next few weeks).
Also, unbeknownst to many people in the international game industry, Bandai Namco has always been a mobile game juggernaut (on the Japanese market) in its own right. The last time the company made a public announcement (in December 2012), it boasted a total of total 30 million users across their social games offered in Japan.
Another reason might be that Bandai Namco and DeNA are simply on a different path with regards to growth. While Bandai Namco added 77% over the last 12 months and is at US$4.7 billion in market cap at the moment, DeNA lost 29% and is at US$3.2 billion:
Via Inside Games