Why are some airports not allowed as IFR alternates, even if conditions there are VFR?
There is a little bit of a misunderstanding in your premise. There is a difference between an alternate airport and a diversion airport. An alternate airport is your listed optional destination for the purposes of filing a flight plan. A diversion airport is the airport to which you head when you can not land at your original destination. They may be one in the same. For instance, when you divert to your alternate airport.
It is not that the airport can not be used as an alternate. It is that the approach can not be used to qualify the airport as an alternate. You can not use the standard precision approach, standard non-precision approach, nor non-standard approach alternate minima. You can, however, use the VFR weather minima to qualify the airport as an alternate. That means that both I19 and I73 are treated the same when filing. This is for filing purposes only.
Once airborne, the situation changes. If the weather or any other circumstance dictates a diversion to airport I19, you can use the procedure to shoot an approach down to the IAP minima regardless of if you filed it as an alternate or even if it qualified as an alternate when filing.  Therefore, you could possibly effect a landing at I19 in IMC, but not at I73 in IMC. You could divert to either one in VMC. Even if your filed alternate were an entirely different airport.