There are restrictions beyond the timing. The Church wants missionaries to initiate any conversations, and asks them to use "good judgment" on the length of calls and chats. They're also told to be respectful of how this affects their missionary companions.

The Church is direct in explaining why it's loosening the rules: the widened communications should help keep families in touch and give a boost to missionaries who are feeling homesick. Indirectly, it's also an acknowledgment that communication technology has changed -- there's a whole generation of missionaries that has grown up with smartphones and may prefer a video call to an email.