Purely a BMW decision to not make this available. The OBD-II buss has voltage information on it and could easily be displayed if BMW choose to.

But I'll tell you why they probably do not with a little story.

Many years ago I was a product planner for a couple of years at a tractor manufacturer. Now, tractors have something called a Power Takeoff (PTO) which runs at a nominal speed of either 540 RPM or 1,000 RPM. One of our competitors decided to go electronic and, while they were at it, put an electronic digital PTO speed measurement in the instrument panel. Well, all of a sudden the dealers were getting complaints that their "540" PTO was running at 537, 546, 533, etc. Well, of course they always were only approximately 540, and it didn't affect performance at all if it was off a bit, but now that they had the precise data, some owners were complaining and hassling the dealer with an unreasonable expectation -- "It's a 540 PTO and it damn well should run at 540!" They soon removed the digital PTO speed measurement from the instrument panel.

Now, I have monitored the voltage on my X5 during the first 6,000 miles and have noticed that the voltage varies from about a maximum of 14.8 volts, to a minimum of 13.8 volts, while running. It seems to have to do with the state of battery charge and temperature, with cold temperatures providing higher voltage, and the lower voltages showing-up after a couple of hours or cruising at interstate speed. I am not 100% sure about the temperature dependency; however, I have seen a rough correlation and such behavior would make sense. I expect that this is normal operation and that the lower voltage happens when the system senses the battery is completely charged "to the top."

But... I also expect that a few non-technical people who owned a BMW would be badgering the dealer if they saw the voltage going from 13.9 to 14.8 without obvious rhyme or reason (to them).

So I suspect the product planners, or their equivalents, at BMW decided to avoid the problem by simply hiding the voltage data, and only having the vehicle inform you if it sees a problem.

Well, at least that's my guess.