Which why are you asking?
Why did they sign him, or why did he choose to sign?
The answer to the first question is because even now, past his prime, he's by far better than AT LEAST 80% of their roster. He's one of the true greats of puroresu in general and junior heavyweights specifically.
As for why did he sign with them? Money, I suppose, and there is still some bizarre prestige amongst the Japanese in working for the WWE.
I do think it's a bad move, though.
I give him one month, TOPS, before they have him jobbing to Albert or Big Show.
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