From the Senate Report:
The Committee found that Manafort's presence on the Campaign and proximity to Trump created opportunities for Russian intelligence services to exert influence over, and acquire confidential information on, the Trump Campaign. Taken as a whole, Manafort's high-level access and willingness to share information with individuals closely affiliated with the Russian intelligence services, particularly Kilimnik and associates of Oleg Deripaska, represented a grave counterintelligence threat.And:
The Committee found that Russian President Vladimir Putin ordered the Russian effort to hack computer networks and accounts affiliated with the Democratic Party and leak information damaging to Hillary Clinton and her campaign for president. Moscow's intent was to harm the Clinton Campaign, tarnish an expected Clinton presidential administration, help the Trump Campaign after Trump became the presumptive Republican nominee, and undermine the U.S. democratic process.
So, tell me again why wouldn't the FBI investigate this? Why shouldn't they?