I'll go ahead and add my answer to your question in the meantime mb.
Two basic reasons these days. One is that some people prefer a voltmeter for various reasons, and the second is that when you upgrade to a higher output alternator (above about 70 amps) the ammeter becomes less than desirable from a wiring standpoint.
To give some more detail, some people are just used to reading a voltmeter as ammeters have not been in regular use since the early eighties. Others prefer the type of system information a voltmeter gives vs an ammeter. Some of those also don't think an ammeter gives the full story (I'm not one of those however). And still some more are just tired of their finicky ammeters not working at all. EB ammeters are fairly reliable as ammeters go, but in other later applications they hardly ever worked.
From the high-output alternator's standpoint, the gauge itself is fine. It's the wiring that is less than capable of higher current flow.
Some ammeter designs are literally dangerous when used with too high a current flow because the current passes through the wiring inside the gauge. Ours are not designed like those however, and are quite safe with handling excess current flow. They're inductive reading (like a timing light) and have no direct electrical connection.
However, and this is a BIG however, the actual gauge wires themselves are not capable of handling all that new-found current reliably. The size/gauge of wire used by Ford in the original harness, and even those of some aftermarket replacement harnesses, is just not robust enough to handle more than about 70 amps reliably.
The wire can handle the current for a short time, and even quite a long time if it's in perfect shape. But after 38+ years of life, it's rarely in perfect shape. And you really don't want to be the one to test that theory.
So that's a long way of saying that if you upgrade your alternator and don't upgrade the specific wiring to a larger gauge, then an ammeter is not recommended and you might as well change to a voltmeter. Especially handy to do while your gauge cluster is out and being refurbished.
Hope that helps. Quite wordy, but at least gives you some info to digest.
Paul