Why must my device be compatible with a certain ROM before I flash it to my device? This it the explanation telling me not to install a custom ROM without checking the device's compatibility. "Now, be careful as each custom ROM comes with a version that is suitable only with one device; you can’t install the same variant of firmware on all the Android devices as the same is not compatible." 

But, I'm using a HP Slate 7, there isn't an official thread for this device on XDA Developers, and the furthest I can go is to root my tablet. Other than that, there are no custom ROMs specifically for this device.

My question is, why can't I install a custom ROM if I follow all the correct processes for flashing a custom ROM, even if my device is unsupported? Is it purely because of the bootloader? Or the kernel? I'm fairly new to Android, but I've been reading up on new terms, so can I have some guidance?
No long explanation needed, the reason is quite simple: Different devices come with different hardware components, such as the camera or sound "chips". Those require special drivers, which are in most cases proprietary and not part of AOSP (which is the "Android base code").

This means, each ROM must include device specific code, and thus is specific to the device it is created for.