Apple failure to warn about lose of dual active phone lines before converting to esim

I found that the iphone 13 and later can handle two esims at the same time …

… why is this not the case with the iphone 12?

Because it ISN’T.

Engineered products don’t just “spring into their final instantiation” all at once. They evolve and mature over time.

eSIM modules and OTA provisioning was still very much a “work in progress” with the carriers.

eSIMS were still fairly new, having been fielded with iPhone Xs/Xr. Carrier support was hit-or-miss.

This is NOT “news” nor was it “concealed” by Apple.

That particular part of this article has remained largely unchanged since the iPhone 13 was released:

Using Dual SIM with an eSIM - Apple Support

“… You can use Dual SIM by using a physical SIM and an eSIM. iPhone 13 models and later also support Dual SIM with two eSIMs, in addition to a physical SIM and an eSIM … “

Re: “… Now I cannot have both lines active …”

OF COURSE you can !

If you need your second line simultaneously active on your 12 …

… just call your carrier back and get a new nanoSIM.

Hi, this is not Apple Support. Apple and Apple Support do not participate here. Everyone posting and commenting are fellow Apple users, just like you. 

Please provide your feedback directly to Apple using the link I shared below.

Product Feedback - Apple

I think you may be mistaken about iphone 12 vs 13. This is what Apple says

”What you need

• An iPhone XS, iPhone XS Max, iPhone XR or later with iOS 12.1 or later
• A wireless network provider that supports eSIM

To use two different network providers, your iPhone must be unlocked. Otherwise, both plans must be from the same provider. Contact your network provider for more information.”

Using Dual SIM with an eSIM – Apple Support

My bad you are right. 12 and 13 are different!