The reason for the meeting was finance. I don't have that research to hand, but France was absolutely screwed for finance. They had a structural deficit and had tried everything (including inventing entirely new and fantastic monetary systems, firing successive finance ministers, etc. There wasn't a mechanism to levy new taxes or to change the allocation of existing taxes. It had been tried, and internal inertia and hostile stakeholders had ruined the effort (Imagine that every bureacrat in France was [Ted Cruz], 2).
update in response to comments. The King could declare a new tax, but unless he went door to door to collect it, he would receive no tax revenue. Tax collectors performed the function their fathers did, in the way their fathers did, and distributed the revenue the way their fathers did. The tax collector didn't work for an institution, he held a sinecure, and there was no real way to remove him from office. (which had something to do with the levels of corruptions which are unimaginable to modern students.) Every tax collector, every judge, every magistrate, every gendarme, every minor elected official had their own version of how the government was supposed to work. (actually they had two - one they paid lip service to, and one they actually carried out day to day.) If the King declared "Water is wet", the entrenched bureaucracy of these street level bureaucrats woudl interpret that as "The King has declared that water is wet, and in accordance with my ancient perogatives and the authority of my office, I interepret that to mean that I should have more Jam on my bread!". And there wasn't anyone in the whole country that could disagree. The king could dismiss you from his presence, and might be able to dismiss you from a top level office, but he didn't control enough offices in the country to make a difference. It was "personal rule" not just because he ruled without governance, but because the limit of his authority was the limit of his personal influence.
There is an excellent source (I've lost the title and reference, but the paper made a deep impression on me) about efforts to reform the French Salt Tax. Can anyone recall?
France didn't have the equivalent of a constitution, or a real legislative branch. France had a perplexing combination of absolute personal rule combined with entrenched customs that could not be altered by any known mechanism.
If I remember correctly, the Estates General was called as a "Hail Mary" play - get the largest possible buy in to the emergency measures necessary to keep the country from disintegrating into anarchy.