Because after 200 years human accents do not change...
An interesting idea. Let's ask around.
"Hey, Scotty, has the Scottish accent changed much since the twentieth Century?"
"Nae, laddy. It's always been about the same, since then, anyhow."
"How about you, Mister Chekov, has the Russian accent remained unchanged since the days of the Soviet Union?"
"Yes, sir. It's just as they spoke back in Leningrad."
"Oh, look! It's the Rozhenkos! Sergei, Helena, would you agree with Mister Chekov?"
"Oh, of course we would. He's such a nice young man."
"Chief O'Brien? What can you say about the Irish accent?"
"I'd say it's well-suited to a good drinking song!"
"And, Doctor Bashir, will you be joining him in these songs? Where does your accent hail from?"
"South Africa."
"Well, hi there, Lieutenant Commander Singh, assistant Chief Engineer of the
Enterprise-D. Watch out for cloud entities on your way to Parliament or you might end up with the dubious distinction of being the first crewman killed on the E-D. By the way, that's a rather thick and luxurious accent you speak in. Is that from India? Like your name?"
"Oh yes. Thank you for noticing."
"Oh, Captain Picard, there you are. I was hoping to ask you something: All these other Earth accents seem totally unchanged for centuries. You and your brother and sister-in-law and nephew all seem completely British in every way yet I'm told you are actually French? How is it that the French accent evolved into a British accent?"
"What's that!?!?" he yells as he points over your shoulder! You turn in terror to see what it could possibly be only to find nothing. You turn back and Jean-Luc is running down the corridor as quick as he can.