Could you correct me if I'm wrong
Your sentence is fine. 我不知道你为什么不常回我信息. However, your English translation doesn't match with your Chinese sentence. 


  I don't know why you take so long to text me back. 


我不知道你为什么不常回我信息. means I don't know why you don't text me back usually. 

The neat way to say "why you don't text me back usually": 


你怎么总不回我信息呢？
你怎么老不回我信息呢？
你为啥经常不回我信息？
你为啥总不回我信息？


The way to say "I don't know why you take so long to text me back": 


你为什么信息回得这么慢！
the one like Pedroski suggested: 我想不通为什么你回复我的信息要这么久！