The 75-year-old was painted as a conniving serial rapist in "Bikram: Yogi, Guru, Predator" -- with several women explaining how Bikram abused his power to sexually assault them.

The burning question at the end of the documentary ... how the hell was Bikram NOT hit with criminal charges???

The L.A. District Attorney's Office acknowledges a case was submitted in 2013 involving 4 accusers -- but at the time "it was determined that there was insufficient evidence to file criminal charges."

Here's the breakdown as to why prosecutors shot down each allegation  ...

Alleged Victim 1 (Claimed she was sexually assaulted at Bikram's home in 2005) -- "No other witnesses were present and no physical evidence was recovered."

Alleged Victim 2 (Claimed she was sexually assaulted at Bikram's home in 2006) -- "No one witnessed the assault."

Alleged Victim 3 (Claimed she was raped twice in 2011) -- "No percipient witnesses or corroborative evidence are available."

Alleged Victim 4 (Claimed she was raped three times in 2010) -- "No percipient witnesses or corroborative evidence are available."

The D.A. also notes the statute of limitations had expired on Alleged Victim 1 prior to reporting.

But, even though those cases didn't go anywhere, the D.A. tells TMZ it will conduct a "complete and thorough review" if law enforcement presents any new allegations to the office.

Our LAPD sources tell us there are no current investigations but as one person put it, "If a victim does come forward, the LAPD will investigate."

Law enforcement sources tell us ... people involved in the previous investigations strongly believe the accusers ARE credible, but it's difficult to get a conviction without witnesses or physical evidence.

Of course, Bikram is currently on the lam ... he fled the country after he was hit with a $6.8 million judgment in 2016 after his former lawyer, Minakshi Jafa-Bodden, sued him for wrongful termination and sexual harassment.

Jafa-Bodden -- who was featured heavily in the Netflix doc -- claimed she was fired after investigating claims Bikram raped a female student.

For his part, Bikram has denied raping anyone -- claiming he doesn't need to rape because there are plenty of women around the world who would willingly have sex with him.