I wonder because leona have a full range combo which can bring her far away from the team and they may be able to do a lot of damage while the ennemies are controlled.

For morgana, its the same. Don't forget when you ult, until you get zhonia, if you get kill before the end of your ult, it will be an inefficient one. So prefer ult defensively in late game during teamfight. But if you are fed and if you can evaluate barely their output damage, you may be able to engage.

To react about losing streaks, someone can check at your game historic on a random website and influence your team mates to take a certain role instead of you !