>>10696397Because they're completely different control systems.
DC, 'direct current', is your basic electrical circuit. Power goes into the track, through the locomotive wheels/pickups into the motor, powers the motor, then leaves the engine through the other side's wheels and track and goes back to the power pack. Every engine on the circuit will get the same amount of power and move at the same time if possible. Multiple independent engine operation is only possible with DC if a layout is large enough to support electrically isolated blocks, and even then only one engine (or consist if multiple engines are lashed up) can operate in one block at any given time.
DCC, Digital Command Control, uses AC current (IIRC) which is fundamentally different and why trying to run a DC locomotive on DCC will just get a buzzing noise. There are some DCC systems I've heard of that CAN run DC with an 'address zero' method but I've heard this also will burn out motors much faster and I've never seen it in action, so it's generally not a good idea. Anyway, the point of DCC is that the AC current is constantly present in the tracks (vs DC where the power pack controls how much power is being applied), with electrical signals being sent to specific locomotives through their decoders. This allows for independent control of both motor functions as well as literal bells and whistles, etc, as it's the decoder in the engine that's modulating the power going to the various components, including the motor itself.
Those decoders usually will have a DC mode that automatically triggers if it detects a DC current, where it just goes into a pre-programmed setting that controls things like sound and lights based on how much power it is getting. Though this usually requires a LOT of power on DC and as such if you're not running in a continuous loop, it's not going to be smooth operation as the engine has to turn 'off' every time you switch directions.
