The "why" involves things we cannot ever know, does it not? The leads come down to what we can infer. Who did Luke and Kei spend time with? Who did they trust? What did they say in trials, and who agreed or disagreed with them? I do not think Franziska was wrong in her path of logic - she did point to Luke with it, though also to innocents - but I can't say I have always paid close enough attention to be certain of the answers.

All we can truly do is say who we find suspicious and for what reasons. The motives those people would have to do the suspicious things are rather hard to say.