Why doesn't the standby mode support all phones when charging?

We can't answer why questions. This is a user to user only forum. All we can tell you is Apple designed Standy Mode to work optimally on iPhones with an Always On Display. Phones which don't have an Always on Display, lack the hardware of phone with an AOD. That doesn't mean you can't use Standby Mode on a phone without an AOD, it just means you have to wake the screen. Otherwise, new features get rolled out every year, which are hardware dependent and older phones or even the newest phone which lacks the hardware won't be able to use. It's just how technology moves forward.

Standby mode does not ‘keep the screen on’ on the 14pro and 15pro. That’s a function of the always on display on those devices.

I suspect it all comes down to power usage. The Always on Display is engineered to be a low consumer of energy and that is a different display from phone without an AOD. It could and I'm just guessing here, prematurely age a screen and battery in an iPhone which doesn't have the AOD, if the screen were lit continuously while on a charge.

I have the same issue.

when my phone is on charge at home in the portrait position the standby clock is displayed.

When my phone is in the same position in the car using Apples MagSafe charger the standby display rarely comes on.

I would prefer to use it in my car as the clock is easier to see than the small one on my dashboard display

I agree. It’d be a nice function to use as a bedside clock, if it’d stay on continuously. I have a 12. It stays on for 15 seconds and then rhe screen goes dark. Pointless to put that on an update for phones that cannot turn it on to stay on throughout the night. 👎👎