I don't know for sure but my thoughts are that, sins Trinityforce have become a better item sins it provides easier positioning and good chase potential and there for many players opt for Trinityforce sins you get great escape with the increased moment speed.
If they fail with the blue Ez it might be that they don't get the tear early enough to fully stack it and sin Muramana is a key item for blue Ez dmg they don't have enough dmg.
It can also be a consequens of that blue Ez don't have great early game dmg and need to get some items to really have a great impact on the game.