Left 4 Dead 2 has more enemy types, more enemy variety, more weapons, more maps, more game modes and more features (Jukeboxes, Crates, Easter eggs)
Then why oh why do I prefer to play the original Left 4 Dead, despite it not having Workshop support, being already built in to L4D2 and Left 4 Dead 2 having more stuff in to? Nolsatalgia, art direction, soundtrack I can't say for sure but I prefer to play a nearly 10 yr old game with less features and less maps.
Which game do you think is better and why?
Imo, L4D2 is by far the better game in every single possible way - like you said it has WAY more content with workshop support filled to the brim with excellent mods and maps by the talented community. But the fact that it has ALL of the content from the first game literally makes L4D1 totally obsolete when you can play through the entirety of 1 with everything from 2. Hell you can even enable mutators to make it play exactly like the first game by choosing the one that disables 2's content if you want to play it just like the first game.
So yeah, there's my answer c: