According to an email sent to all staff:

Eliminating the “Reply to All” function will:

• Require us to copy only those who need to be involved in an e-mail conversation
• Reduce non-essential messages in mailboxes, freeing up our time as well as server space

We waste so much time reading emails that aren’t really intended for us or have no relevance to our daily lives and though I understand that this move was because of a cock up by an employee last year, I hope it sets a prescience.

Source: techcrunch