SMTP is used to send emails whereas POP3 and IMAP are used to access email accounts.
While I can understand OAUTH being mandated for POP3 and IMAP for security purposes, requiring OAUTH for SMTP would be problematic. For one thing SMTP is used within the Internet to forward emails between various email servers and there are thousands of applications that need to be able to send emails for which implementing OAUTH would be problematic.
What OAUTH does basically is require an application to obtain an application identifier and then a browser request that the user sign on to their account and authorize that the application has access to some aspects of their account.
From a smartphone or desktop "App" perspective, requiring OAUTH is not a problem to implement as you have access to a browser. However for systems that use email to send alerts, forward voicemails, or other notification types of messages there is no easy way to exchange the authorization information -- thus we are not surprise that SMTP will not require OAUTH. It may still be desirable but to mandate it would require changes to anything that potentially sends emails, from home security systems, phone systems, monitoring equipment, etc.
Enforcing OAUTH on POP3 and IMAP access does however make sense and will not impact as many systems.