Because paying cash to obtain gear:
1. Trivializes the accomplishment of obtaining a full set of best-in-slot space parts.
Your results from hours of gearing up are no different than someone who bought it day 1 for cash. Gearing up and competing with your friends to gear up is an important part of end-game. Many players derive enjoyment from competing with your guild mates to see who can be the first to beat content. Pay to win deprives you of that joy.
2. Trivializes the accomplishment of obtaining a full set of best-in-slot raid gear.
Again, many players compete to see who can be first to get full sets of end-game gear. The combination of end-game gear being craftable and the materials being available most easily for those that paid real-life cash, serves to cheapen this competition. It's like competing in a race where those that give a bribe to the race officials are given a head start. Pay to win deprives you of the joy of healthy competition.
3. Pay to win screws space ship part crafters
Cybertechs make ship parts. There are schematics for some of the new best-in-slot space parts. The crafting material requirements are inflated for these parts to make them harder to craft, yet they are are made easily available for real life cash. The high cost of the materials to the space parts makes it extremely difficult for a crafter to compete with a the real life cash cost.
Pay4BIS screws people who play PVE for competitive reasons.