Because that would draw even more attention, and people will be even more suspicious about what's the meaning of D. and
why the government is after them. The idea is covering it up completely since everyone already forget about it, and leaving it alone, since no one even Nico Robin, who can even read the ancient language still doesn't know anything about it. Oden too only learns when they reached Laugh Tale, and Whitebeard only learned from Roger, after Roger returned from Laugh Tale and become the Pirate King. Before no one knew anything about it.
Not even higher ranks in the marines and the higher ranked government officers knows about it, some marines don't even know about Xebec, let alone D.'s history, probably only some special higher ranked Celestials and Imu knows about D. fully, and former Roger pirates except for Shanks (unless Rayleigh or others told him too after they return from Laugh Tale).
Gorousei is also smart enough to know that starting a witch-hunt against D. would create suspicions and rather than make it more obvious why they were their enemies, they probably thought ignoring it completely is a better idea. Because even D. people like Law, Saul, Luffy has no idea what it means.
Just imagine; Garp, a D., is one of the strongest marines of the world government, why would anyone think that D. are the enemies of the government when they look at Garp? That covers it up.
Or, it could be for political reasons like inviting and accepting Jinbe into Shichibukai, to make peace with Fishman people instead of keep fighting with them like they did with Fisher Tiger. So perhaps they wanted to make peace with D. by acceting them into the marines and Shichibukai.
The question is more why they accept them into the Marines.
Or to Shichibukai. Probably for the reason I explained above, trying to make them look like their strongest allies so no one would think or suspect that D. would be their enemies, and they wouldn't start to think D. was their enemies, or just like trying to make peace with them like they did with Jinbe. They also accepted Blackbeard into Shichibukai, and previously invited Ace to the Shichibukai as well.