Two that I can think of; lower ammunition cost and lower recoil.
On the other hand, the .40 bullet may fail to expand, but the .45 will not shrink.

I am not certain about the reasons for the FBI decision to go to 10 mm, which came first, rather than .45. I have heard lots of various opinions, perhaps someone here has the facts.
IMO the decision is clouded by the “fog of time”. Wanting something new, not the obsolete 45acp probably was high in the bureaucratic mind.

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Ipsis Rebus Dictantitbus