Why don't they just use 440 vac capacitors for all condensers and get rid of 370 vac capacitors?

Because when a fan motor (or compressor), is designed to operate on a 370 VAC run capacitor, that will be its optimal efficiency operating design. We're talking about capacitors used on an A/C in a typical 230 volt power supply to a residence or business. If you substitute a 440 VAC run cap in place of the 370 cap, the motor will seem to operate just fine. However, if you put an amprobe on the motor, you will find an increase in the amps it is drawing. This translates to increased heat in the windings, and some motors can handle a small heat rise while others have a problem with it. It partly depends on the airflow over the fan motor along with the quality of the motor, but in any event, you are decreasing the life of the motor along with paying for the electrical current you're wasting, and also, not using the specified cap the motor calls for, in most cases, will void the motor's warranty. The worst place to substitute a 370 VAC cap with a 440 VAC cap is on a compressor. The increased heat load in the windings causes an increase in head pressure and on some compressors already operating at high head pressure and heat load, the compressor can trip off and will stay off until the shell cools down. Of course, the additional amp draw in the compressor will really drive up the power bill. Bottom line, use the cap the motor specs call for. If you have to substitute to get-by because you don't have the right one on your truck, go back the next day and put the right one in. George Henne--Retired A/C Tech----