Lemme be blunt. Some people believe that since this happened at a time military tensions between Iran and the US were escalating, the plane could have been taken down mistakenly by a trigger-happy general as a false positive.
Perhaps. But a general of which side? It was, I think, over Iran, so Iran's air defense would be the logical suspect. And Iran withholding the flight recorders would fit with that.
The more paranoid, of course, will look beyond the logical suspects...
Last time a civilian aircraft was shot down over Iranian airspace it was the US. And while they agreed to some “no-blame” deal including a financial compensation they never actually apologized for murdering 290 innocent people. https://en.m.wikipedia.org/wiki/Iran_Air_Flight_655