Why do people run the Japanese version for the All-Stars categories? I'm curious because I couldn't find any other major differences than entering a game is a little faster.

In the Japanese version, there's a glitch that lets you instantly warp to the right side of the flagpole when grabbing the flagpole in certain stages, which saves time due to Mario not needing to turn around when getting off the flagpole. It's generally called backwards flagpole because Mario gets on the other side of the flagpole early. The reason this happens is a bit technical (a decent explanation can be found here: https://www.speedrun.com/smb1/thread/xsghm ) but hitting a block containing a power-up can enable this glitch in certain stages, while in others you will automatically be set up for the glitch.

Also, if there was any confusion, backwards flagpole in SMB1 is exclusive to the Japanese version of All-Stars, but there's an unrelated backwards flagpole trick possible in The Lost Levels which can be done on any version of All-Stars.

Thanks for the explanation!