There are various reasons but perhaps the biggest reason is reliability of the troops and the size of Paris.

Reliability of troops

In the 1780s, the government introduced the Segur ordinance to improve the competency of the troops, these ordinances made it harder for commoners to rise in the ranks since the ordinance made it so that only hereditary Nobles whose nobility went back 4 generations could become officers. The only way commoners could get high ranking positions was if they had earned the Cross of Saint-Louis. This caused disturbances in the army ranks since the military was one of the few ways that provided uneducated or poor commoners with social advancement (future Revolutionary Saint-Just's father had been ennobled because of his service in the army, though his father had also earned the Cross of Saint-Louis and worth noting that this nobility was not hereditary so Saint-Just himself was not a nobleman). Another thing that caused major disturbances was the decision to pay soldiers with paper bonds in 1788. Paper money in those days was distrusted and this understandably led to resentment and loss of discipline in the ranks. Thirdly the rank and file of the army was filled with commoners who shared the same grievances and most importantly agreed with the Revolutionaries demand for meritocracy. The Storming of the Bastille provides a few examples of this. Prior to the Storming of the Bastille, the Parisians had tried to break into the Hotel des Invalides, the Governor of Invalides was scared of just such attack and had ordered his soldiers to dismantle the muskets, however the soldiers went about their task quite leisurely and had only dismantled around 20 muskets in 6 hours, secondly during the Storming the Bastille Governor de Launay was hoping for French guard to come to the defense of the Bastille, but when the French guard did show up to the Bastille they sided with the people of Paris rather than de Launay.

The Size of Paris

Paris had a population of 600,000 - 750,000. This was a huge number, far bigger than any army in the world at that point. Attacking Paris would threaten an all out Civil war, but most importantly a city the size of Paris (once they were hostile) could not be held down even if the troops had been reliable.

Here is what historian Hilaire Belloc thought about subduing Paris "It behoves us here to consider the military aspect of this definitive act from which the sanction of the Revolution, the physical power behind it, dates.

Paris numbered somewhat under a million souls: perhaps no more than 600,000: the number fluctuated with the season. The foreign mercenary troops who were mainly employed in the repression of the popular feeling therein, were not sufficient to impose anything like a siege. They could at the various gates have stopped the provisioning of the city, but then at any one of those separate points, any one of their detachments upon a long perimeter more than a day’s march in circumference would certainly have been attacked and almost as certainly overwhelmed by masses of partially armed civilians.

Could the streets have been cleared while the ferment was rising? It is very doubtful. They were narrow and tortuous in the extreme, the area to be dealt with was enormous, the tradition of barricades not forgotten, and the spontaneous action of that excellent fighting material which a Paris mob contains, had been quite as rapid as anything that could have been effected by military orders.

The one great fault was the neglect to cover the Invalides, but even had the Invalides not been looted, the stock of arms and powder in the city would have been sufficient to have organised a desperate and prolonged resistance. The local auxiliary force (of slight military value, it is true), the “French Guards,” as they were called, were wholly with the people. And in general, the Crown must be acquitted of any considerable blunder on the military side of this struggle. It certainly did not fail from lack of will.

The truth is (if we consider merely the military aspect of this military event) that in dealing with large bodies of men who are (a) not previously disarmed, (b) under conditions where they cannot be dispersed, and (c) capable by a national tradition or character of some sort of rapid, spontaneous organisation, the issue will always be doubtful, and the uncertain factor (which is the tenacity, decision and common will of the civilians, to which soldiers are to be opposed) is one that varies within the very widest limits.

In massing the troops originally, the Crown and its advisers estimated that uncertain factor at far too low a point. Even contemporary educated opinion, which was in sympathy with Paris, put it too low. That factor was, as a fact, so high that no armed force of the size and quality which the Crown then disposed of, could achieve its object or hold down the capital.

As for the absurd conception that any body of men in uniform, however small, could always have the better of civilian resistance, however large and well organised, it is not worthy of a moment’s consideration by those who interest themselves in the realities of military history. It is worthy only of the academies."