Because it requires more resources on their part.

POP is remarkably simple. The paradigm is to store messages until the user requests them, then delete them. This can be modified, but that's the general idea. There are very few POP commands. They amount to ``show me what you've got'', ``send me that message'', and ``delete that message'' (beyond auth commands). There are two optional POP commands -- one that allows the client to see only the first portion of the message (defined in bytes), and one that provides unique identifiers for messages, which can allow for better synchronization.

IMAP is, at least relatively, very complex. In addition to the basic commands that POP has, IMAP allows retrieval of portions of messages defined by MIME sections, multiple folders per user, assignation of flags (even arbitrary user-defined ones) to messages, on-server full-text searches (which is useful because you might not have all of the messages on your client), etc. The paradigm is also quite different. IMAP expects that the user will leave all messages on the server and deal with the messages ``remotely''. As such, good clients will fetch only the bare minimum of the message to show in list mode and get the rest only when needed. In fact, there is a way to get the server to send the message headers for all (or any subset) of the messages. Which is not to say that it's impossible to download everything to your client. It's just not what it's best at. But most IMAP servers have ancillary POP3 servers, as well, that read from the same spool as the IMAP server for those clients that are trapped in POP-world for some reason.

In summary, a quote from the POP3 RFC seems in order: ``In short, the philosophy of this memo is to put intelligence in the part of the POP3 client and not the POP3 server.'' So basically, ISPs can either set up POP, which is a very simple message delivery protocol, or they can set up IMAP, which tends to require more storage space, and much more processing power. And since they have no requirements to meet, other than simply getting the mail to you, it's a no-brainer for them. Interestingly, Apple's iTools free email (@mac.com addresses) are supplied as IMAP accounts, though.

To read more about IMAP vs. POP, read Comparing Two Approaches to Remote Mailbox Access: IMAP vs. POP and Message Access Paradigms and Protocols, or visit your local library.

Oh -- Cyrus is my favorite IMAP server. It has many nice features, including quotas, mailbox sharing via very precise ACLs, direct delivery to folders other than your INBOX, and a lot more.