Way can you only use this legendaries in the battle subway ?

Question

The legendary bird trio (Articuno, Zapdos, Moltres)
The legendary beast trio (Entei, Raikou, Suicune)
Latias & Latios
The Legendary Golem trio (Registeel, Regice, Regirock) plus Regigigas
The Lake Pixie Trio (Mesprit, Uxie, Azelf)
Cressalia
Heatran
The Musketeer Trio (Cobailion, Terrakion, Virizion, but not Keldeo)
The Kami trio (Tornadus, Thundurus, Landorus)

Answer 1

It seems not the final legends (Rayquzza,zekrom,mew,reshiram,dialga etc)

Since they are very over-powered, but they seem to let you use the 'smaller' legendaries.