It's because you're logged in as an existing user. Just use a different email address to get a new certificate and sign up with that to get the discount. Then after you get the cheaper certificate, ask support to merge the accounts.
But that's off the point. Regardless of the workarounds, whatever the goodwill DigiCert has accumulated with the developer community over the years, they are actively squandering it now and they don't seem to care.
It was still there in October. So this is a recent thing.