If registered mail is sufficient and that only requires proof of delivery/receipt, why would the same thing for email be insufficient?

Email with proof of receipt (e.g. Outlook's read receipts) would be fine. But sending an email and hoping for the best doesn't meet that condition.

A read receipt is not proof of receipt but proof that you read it. They are not the same thing. If your office receives registered mail but your secretary threw it away without you reading it, you're still legally served right?

Agreed, but there's no delivered (but not necessarily read) receipt that applies to email so that was closest I could think of that counted. The overall point remains: sending an email, with no further evidence, does not count as proof of delivery (all the way to the inbox).