Sen. Richard Burr and the coronavirus insider trading scandal, explained

Why is the DoJ just targeting Burr so far?

Russia!

The Senate Intelligence Committee has investigated the Russian affair and released data that supported the Mueller Report. They were due to release the last portion this summer. That portion covers connections between the Russians and the Trump family and organization.
From the article:
"Last May, Burr’s committee issued a subpoena for Donald Trump Jr. during an investigation into links between the Trump campaign and Russian operatives. The subpoena became an issue for Trump-loyal Republicans who were trying to dismiss investigations into Trump campaign links to Russia as politically motivated. Trump has frequently lashed out at Burr in response, even retweeting criticism of the senator over the Russia investigation as recently as Monday."

By forcing Burr out, they can get a friend of Trump in as Chairman and quash the report.