I created a campaign to start with friends. I own the Player's Guide and Xanathar's but the campaign is only showing the Player's Guide as the content that's shared with no option for Xanathar's.

Do you or another player have a Master Tier's subscription?

If yes, has it been enabled?

If Yes, have you/they tried disabling content sharing and re-enabling it?

If Yes, is the character you or a player is wanting to use actually in the campaign - was it made/added by using the campaign's Join link?

If yes to all, you have a bug. I'd recommend raising a Support Ticket using the Contact Us link found at the bottom of any page on this site.

You could try going here: https://www.dndbeyond.com/account/marketplace and seeing if it gives you an option to resync Xanathar's. If it does give you that option, do that. That presumes that Cyb3r's advice doesn't pan out, but before contacting support. That's another possibility to check.