Comment by p_l

It started out as 16bit enhancement to previous product, and evolved into being a 32bit architecture over development time.

The separation of Data and Address registers are also result of how it evolved over time, AFAIK, ultimately because it allowed to make the CPU cheaper/easier to make. Another element is that 68000 at least has two layers of microcode - first microcode engine generates instructions interpreted by second microcode engine which finally actually drives execution units.