My guess the only reason is this: Because openforce was under heavy time pressure when they had to deliver dotlrn and didn't want to bother with understanding and potentially having to fix the existing package acs-mail.
Don't know what packages are using acs-mail right now, but if we merge acs-mail-lite into it that would mean that at least we would have to change the calls in all packages that use acs-mail-lite, which is mainly notifications. And maybe some more in dotlrn.
Maybe the merge would also suggest improving / modernizing the acs-mail api which would mean that existing acs-mail calls would need to be modified as well (I'd volunteer for that). Or we can build a compatibility wrapper that maintains the old acs_mail_lite:: and acs_mail calls.