Bourn strained his hamstring early on in spring training, and has spent the last week and a half rehabbing in Columbus and Akron. That rehab was slowed somewhat by bad weather (and weather will delay his return by another day, as tonight' contest with the Tigers has been postponed due to freezing temperatures and possible snow), but he has been cleared by doctors to return to everyday action.
Morgan did a wonderful job while filling in, hitting .348 with a .484 on-base percentage, and I'm sure there is a large segment of the fanbase that will be upset with him being demoted. It's important to keep in mind what a small sample (33 PA) of performance we've seen from Morgan, and to remember that he was so bad in 2012 (an OPS+ of 63 in 322 PA) that he couldn't land an MLB contract for 2013 (he played in Japan instead). I'm thrilled he did so well, but to start him when Bourn is healthy strikes me as ridiculous.
I would have been happy to see him remain on the roster, with Elliot Johnson being sent down instead, but Morgan wouldn't have played very much anyway, and because he had an option remaining, the Indians get to hold onto his rights, and can bring him back up if Bourn (or any of the Tribe's other outfielders) suffers an injury.