Thing is: Tsukuba is a rather small track, easy to see a lot for the spectators and is hugely popular. It's also the track where the whole Time Attack Super Lap thing started, so...
Plus, if you want to go really fast around the track, you need a very powerful car to rocket away from the hairpins, yet it has to be 'agile' as well, because Tsukuba has the size of an outgrown kart-track...
The choice for Tsukuba is more straightforward than you might think and anyway, there is a Super Lap tournament where they do all the major tracks in Japan, like Fuji, Autopolis, Odaiba... The Tsukuba thing is only as big asit is because of the massive coverage by Rev Speed, Hot Version/Best Motoring or Video Option (ao).
Bit of a messy reply but I hope you get my point