Is there a reason why no one uses the Famicom disk system to run Metroid? I ask because i honestly see no difference in time advantage.

It may just be preference but i would like to know as i have not really seen any runs on the Japanese version of the game.

Not an official speedrun.com answer, just my thoughts:

1. Foremost, the FDS metroid has different RNG which result in a greater variety of enemy and enemy projectile patterns. The FDS-troid's patterns are also closer to true randomness, changing patterns as often as each cycle, vs. NEStroid which basically sets one cluster of enemy patterns on startup/reset/death. These meaningful gameplay differences imply that it probably should be treated as a separate game for a fair comparison of WR times.
2. I don't believe Metroid1 is as popular in Japan?? E.g. on nikoniko.com, the japanese youtube/twitch, I usually can't find more than one (or any) videos of FDStroid gameplay even if I search metroid メトロイド in kanji.
3. Might be a pain to do runs on original FDS hardware b/c the belts were designed to break after 1.47 seconds of use? ;) ETA: In short tho, it's a good question but it just doesn't come up as an actual issue that much. ¤shrug¤