I know a little of the pre-Revolutionary situation in France:

By 1780~, the French nobility was firmly split into two camps- the nobility of the sword (I don't recall the French) and the nobility of the robe. There existed other classes of noble, but I don't know enough about them, and my sources do not elaborate. It is entirely possible that my explanation of the noble classes will be insufficient, and I'd be delighted to have someone clear up misconceptions regarding these terms of reference.

The nobility of the sword were those whose titles had been granted prior to the end of the 15th century- their name, as far as I am aware, is a reference to their right to wear a sword in public.

The nobility of the robe were those ennobled after the 15th century, for whatever reason. Under Louis the XIV, they had become more prevalent in the French system of government than the nobility of the sword, as they were much more frequently wealthy than the nobility of the sword- though I will get to that. Nobility of the robe, most importantly, could be bought. The number of titles available for sale varied annually, but was fundamentally up to the monarch- in times of financial hardship, the French crown would sell a large number of these titles, inflating their value and irritating existing nobility. The titles were sought after as a mark of prestige- a historian whose name irritatingly escapes me as I donated my books recently (I suspect it was either Doyle or Mcphee, cited in Liberating France)- stated to the effect that "all men believed they had a right to nobility- for the rich, it was because they were powerful, for the old nobility, it was because it was their birthright".

Now, to answer your question- the sticking point here was that the nobility of the sword were frequently impoverished- whilst ownership of a fief was a perquisite for the title, fiefs neither had to be reasonably sized nor wealthy to justify the title. Many nobility of the sword, by 1780, were as poor in terms of income as a landowning peasant, sometimes worse. For an interesting, if somewhat romantic take on that particular aspect of society, have a look at the film Ridicule. A key reason for this impoverishment is that nobility were barred from participating in industry- and thus were unable to take part in the largest area of wealth creation in the country at the time.

The nobility of the robe, however, were not initially subject to this restriction- they were frequently merchants/industrialists, some of whom would have been participating in the lucrative slave trade or other colonial activities, who bought their way into the nobility. The offices were not cheap- often in excess of 100,000 livres (for comparison, the entire French budget was only 300~ million livres in 1781). For this very reason, only the richest bourgeois became nobles of the robe, to carry out duties such as tax farming and sitting in Parlemonts (law registrars) across the country.

These individuals were subsequently viewed as inferior by the nobility of the sword- who believed that the nobility had a right to be wealthy, and that the sales of venal (purchasable) offices and titles diluted the purity of the French nobility. Indeed, they had a right to be worried- the nobility of the robe became far more powerful than the nobility of the sword as the ancien regime drew to a close. By 1788, the nobility of the sword were so alienated from the government of France that they refused to ratify financial reforms at an Assembly of Notables convened in 1787 by King Louis XVI, effectively allowing France to go bankrupt and catalysing the Aristocratic Revolt. (This is not the only reason, but it is the only reason relevant to your question). For this, Schama calls them "the first revolutionaries".

To conclude: In pre-revolutionary France, the financial situation of the French monarchy and the decline in the profitability of land ownership led successive Kings to sell nobility to rich commoners. This created tensions between those burghers and industrialists who aspired to nobility and the existing stratum of nobility they would replace in importance.

Sources:

The French Revolution, Peter Mcphee

Liberating France, Fenwick and Anderson

The French Revolution, Christopher Hibbert

The spirit of change : France in revolution, Morcombe and Fielding

(I must confess, I am not impressed with the quality of this answer- I would have preferred to offer you additional quotes- but I hope it answers your question somewhat.)