To let him display his powers before the final conflicts starts. Unfortunately for him, he just showed up when the Straw Hats were already really strong, when Momo needed to prove he can do something to protect his nation and when Shanks needed to get more hype after only showing up every once in a while (though even with just that, had quite a lot of hype).
He was still impressive and didn't even use that sword of his and I'm sure he'll do much more in the future. Aramaki doesn't exist own his own, his involvement was always gonna be affected by the current state of the series, with Luffy and his allies having progressed so much and needing to prove themselves and Oda wanting to involve Shanks after claiming he'll do stuff soon.