The primary reason that they have not co-existed is that they have not shared the same timeline and the same paradigm as marketing channels that people should focus on.
Pre smartphone(before2007 and the iPhone) was the era of era sms marketing. Everyone read and cleared their inbox and companies inundated and over exploited the channel with marketing messaging
This was also the time when people were doing their email marketing primarily for the desktop.
The advent of iMessage and other messaging apps such as Facebook messenger meant that fewer people checked their phone inbox
The radical shift in email came with the coming of Web 2.0 in 2010-2013. People started using their phones much more during this period and email marketing started to focus on being optimized for mobile. Tools such as MailChimp managed this transition quite well and have done well with web 2.0
Get the full story here: