When I was looking into using the CPAP, I did check out some battery-operated models.
The first thing to note is that the batteries (for the models I saw, at least) were all extremely heavy and bulky -- we're not talking about 4 AA batteries here. Also, for the number of hours that I'd be in the air, I would have had to recharge the batteries, which meant lugging along a battery recharger. And, unlike the CPAP machines, not all of the battery rechargers were compatible with non-US voltages. Plus, of course, you need to bring the regular AC plug-in equipment.
If I recall, another issue with a few of the battery-powered models is that some used batteries that were verboten on the airlines at that time, presumably because of whatever is inside of the battery.