Chocolate Doom attempts to replicate the original Doom experience in a modern source port, without the embellishments, bug fixes, and features offered by source ports like GZDoom. It's like the difference between building a modern clone of the Amiga 500, hewing as close to the original designs as possible, and building a "modern Amiga" with a PPC CPU, modern GPU, AmigaOS 4.x support, etc. Sure, the second is neat, but some people want to work within the exact constraints and quirks of the first.