I can make a guess on why they removed POP. Its a VERY old technology and is designed on the idea of moving the email to your local system.
For example, searching all your POP emails requires a local copy of them all. On a thin client or mobile platform, this can take up Gbs of space. Personally, I have 1.5Gb or email.
The IMAP protocol allows server side searches that return a list of message ids. Then the headers are all that are pulled down. Clicking on one makes the system get the email in question, not a copy of everything.
IMAP only stores a few days worth of headers (usually) on your system. POP requires all of it.
POP is highly inefficient with network traffic normally.
IMAP also supports additional features that are very helpful on a mobile environment, such as IMAP IDLE (sometimes called push), which permits near instant notification of new emails. POP requires regular checking of the server asking if there is anything new to download.
IMAP also allows you to pick which folders are downloaded and which are not. Additional issues with folders are, if I get an email from POP, move it into a folder on my mobile system, my home one will not see what folder I moved it into.
Overall, IMAP is far superior, especially on a resource restricted or mobile platform.
So the reasons for dropping POP are simple, IMAP is a better solution and forcing its use makes for a better experience.
And before anyone whines about "Microsoft shouldn't make me use the system in a certain way", think back. People whined about the downplaying of the command prompt then Windows 3.1 and Win96 first came out.
"Who needs a mouse? I can do everthing easier and faster with a command prompt."
This also goes along with the "the internet is just a fad", "no one will want a smartphone", "why would anyone want a pc without a keyboard" and the list goes on.