While no poster here can know the reason, here are some possible reasons, ordered in what I think are the strongest to least strong:
1) The Celtics front office thinks Morgan is a better player and/or doesn’t like Diallo in general. For all the raving that occasionally pops up about Diallo on these forums, it should say something that he’s been with the Detroit G-league team all year, and Detroit, a tanking team that’s looking cheap talent to add going forward, brought in Diallo for a Covid 10-day and then let him go, and no other team has shown interest.
2) Morgan was with the Celtics in training camp and has been with Portland all year (save for a brief Covid 10-day of his own at one point), and thus knows the Celtics system better. If you’re pressing a player into emergency service, the player who knows where to go on the court is likely to better than the play who doesn’t, even if that other player is marginally better (which, again, he may not be, but we can concede the point for this particular supposition).
3) Perhaps Morgan was immediately available whereas Diallo was not. Morgan signed a 10-day contract (not a rest-of-season deal) as the Celtics knew they were about to be down four starters for a game in under 24 hours. They needed a body, knew they could get Morgan to Toronto in time, so the Celtics just went that direction instead of getting ahold of Diallo’s agent, waiting for Chieck to make a decision, etc. Sometimes you just need a body immediately, and Morgan was the best of those bodies that were immediately available.
4) The Celtics have now signed multiple players from Portland and as well as their training camp to NBA deals. Perhaps this is a conscious decision for the future to help entice players to join their camp and/or G-League team, because they can point to it as a reasonable way to get to the NBA later in the season if it doesn’t work out in October. Most players of training camp invite quality have more than one offer, and they’re going to pick the one that gives them the best chance to get to the NBA.
I think 1 & 2 are the most probable reasons, but 3 & 4 could have been contributing factors.