A quick Google search of "Mexican Restaurants in Tokyo" brought up over 30 results in Tokyo. Hell, the other day I was watching the original Iron Chef from the 90s, and they brought in a Japanese Mexican Chef as a challenger.
I might have expressed myself wrong, it's not that there aren't Mexican food places at all, it's that they're much more rare than in the west (and the US specifically), and usually not great.
Plus, 30+ restaurants in a city as gigantic as Tokyo is not much at all.
Even if it were hard to find a good restaurant nearby, Greater Tokyo is fucking HUGE! I would be shocked if you couldn't find pretty much all cuisine in or around Tokyo.
I was in Japan this year and didn't see any Mexican food in the 3 major cities I visited. I'm not saying it does not exist but it's definitely not common if it does.
That also makes sense, though. Food in Japan, even the foreign food, has a specific palette it's targeting. Mexican food is extremely different from Japanese food.
Typically, any food that is introduced to a new culture is successful once it's adapted to that cultures palette. Any Mexican food being successful in Japan would likely be more akin to the Mexican/Asian fusion places we have in the States than traditional Mexican food.
Tangential anecdote: when I visited San Luís Potosí, I ate several meals at a place called Café Tokio. It was good but there was nothing Japanese about it beyond the name.