The primary use-case for dpaste.com is ephemeral content — code snippets which will be shared shortly after posting, and maybe never seen again. Other uses merit longer-lived items, but it is not intended to be a publishing service.
The maximum expiry is 365 days.
The major reason not to have a "never expire" option is spam. The site is very actively hit by spammers. I have anti-spam automation that detects most spam, but even spam misssed by that process expires eventually. If "never expire" was an option, spam would be even more of a resource drain and maintenance burden than it already is.
Also, some legitimate users create hundreds or thousands of items in a day. Many of these items are set to the maximum expiry already. If there were a "never expire" option, many of these user would surely choose that instead, and the database would grow without limit.