Why is there a Conspiracy theory and what is it?

There is not one conspiracy theory there are possibly 100s just search online for them, here are a few.

The Driver was drunk

MI6 had planned it

She was about to marry an Egyptian Muslim

She was about to have the baby of a Egyptian Muslim

Diana’s body was deliberately embalmed soon after her death to hide evidence

There have been reports of a bright white flash just before the car entered the tunnel, blinding the driver.

Analysis of the wreckage of the Mercedes revealed it had glancing contact with a white Fiat Uno car which left traces of paint on the Mercedes bodywork but extensive attempts by the French police to find the vehicle involved failed.

The essence of the allegations made by Mohammed Fayed were the white Fiat Uno was used by the "security services" to block the road in front of the Mercedes, causing it to swerve and thereby crash into the side of the tunnel.

Fayed further alleged that the Fiat Uno involved was owned by a French photojournalist named James Andanson

James Andanson committed suicide in May 2000. His body was found in a black, burnt-out BMW in a forest in the south of France, the doors were locked with no sign of the car keys.

The absence of CCTV images showing the Mercedes’ journey from the hotel to the crash site has been frequently cited as evidence of an organised conspiracy.

There was some media discussion in April 2006 suggesting that Diana was a faithful seat belt user and therefore the fact that both her and Dodi’s belts either failed or were not used was sinister and may suggest sabotage.

The first call to the emergency services switchboard was logged at 12.26 a.m. The SAMU ambulance carrying the Princess arrived at the Participatory Hospital at 2.06 a.m. This length of time has prompted much conspiracy-related comment.

The conspiracy theory grew out of shock, people struggled to believe that someone so young so well known could be snatched away like that.