I believe it was a combination of factors. For one Louis XVI dithered too much and never gave his whole hearted support to serious reform, so the aristocracy was less inclined to give way on the issue. A lot of time was wasted trying to bring around the Assembly of Notables (whose members had been hand picked by the King and Calonne and yet who still resisted) while French finances continued to deteriorate. Lefebreve called it the first capitulation because 'the King was consulting the aristocracy instead of notifying it of his will'.
Then when reform became a matter of necessity Louis was confronted with the opposition of a patchwork of 13 different provincial Parlements. This came out of the fact that reforms in taxation went hand in hand with reforms in administration that would have curtailed some provincial rights in favor of a more streamlined and thus more centralized government. Overruling them all would have been a long and exhaustive affair. This was the rub, the reforms called for were deep structural reforms not just the imposition of a new tax and the aristocracy and the clergy forsaw the ways it could threaten their positions and enhance the power of the King so for once they dug in their heels and for the first time since maybe the Fronde they proffered real resistance.
When the French state became insolvent it was deprived of strength had little leverage to use against he Parlements. The Parlements led, I believe, by the Parlement of Paris issued an interpretation of the Fundamental Laws of the Kingdom that simultaneously pronounced their opposition to the reforms and also essentially abdicated their competence, ie they said no and then said that even if the King overruled them it was moot because neither the King nor the Parlements had the authority to do anything without consulting the Estates. Louis XVI did in fact attempt a de facto lit de justice in 1787 under a 'royal session' of the Parlement of Paris over which he personally presided but Orleans led the opposition against him and it broke up with declarations that the King's actions were illegal and his registration was null and void. This sparked serious rebellion among the other Parlements and the aristocracy in general in which they disclaimed the power of the King to act outside of consulting the Estates per French tradition.
In reality it was a power play by the aristocracy to curtail absolutism in France and weaken the monarch by taking advantage of the financial crisis. The first and second estates were willing to submit to greater taxation but only in exchange for more political autonomy and guarantees on their traditional privileged status. They saw the Estates as a vehicle to gain back those rights from the King, obviously things didn't work out as they anticipated.
So I suppose the short answer is if Louis XVI was more determined and showed a little more steel he could have, with difficulty, forced through reforms in 85-86 but by 88-89 it was too late to force the issue by royal fiat.