Some stories I've read have had tokyo (and in particular that part of the city) to be located on an upwelling of natural magical power, making it easier for Youma to survive/open the gateway from the Dark Kingdom/the locals have much larger energy reserves to draw from/etc. The exact reason varies from story to story even when the same 'upwelling mana' power source was used.
I'm unsure if there was any official reason for why so many SM villains showed up in Tokyo (Dark Moon Family and Deathbusters at least had reasons to set up in Tokyo)
I'm unsure if there was any official reason for why so many SM villains showed up in Tokyo (Dark Moon Family and Deathbusters at least had reasons to set up in Tokyo)