I don't think Moira being seen as a worse secondary healer has to do with her inability to 'keep up' with dive. With her dash every five seconds and ability to throw out healing/damage orbs, she should keep up with dive fairly well.

I think it might be because usually, since Mercy is a must-pick, you need some defensive ult from your secondary support and Moira's is very lackluster. It only projects in a straight line and leaves Moira fairly vulnerable to any type of CC (unlike Zen who goes invincible or Lucio who has his high mobility and also gets his extra HP). Additionally, she offers very little utility compared to Zen's discord orbs (which are huge) and Lucio's speed buffs.

I wouldn't say any of that makes her a "throw pick," though.