Two issues make no sense to me why Microsoft would want to acquire Automattic / WordPress.
1) WordPress runs predominantly on LAMP servers. I just can't vision Microsoft owning anything which directly or indirectly promotes Linux, Apache, MySQL, and PHP.
2) WordPress is licensed under GNU GPL. If Microsoft were to acquire Automattic / WordPress and decided to switch to a proprietary license, I believe a fork would occur and the software would live on under a new name.
Thanks SiberianHuskey for an interesting read.