Because sales tax on a product, a requirement to purchase a fixed amount of a good per person, and a set fine for violating a law all hurt poor people a whole lot more than rich people.

A gabelle or indirect tax is basically what we think of as a sales tax. It taxes consumption instead of real property held. In the Middle Ages, when the salt gabelle like the one we're dealing with here were born, that meant land. But governments trying to generate fresh income from property taxes faced two problems: one, reluctant nobles and aristocrats ("reluctant"); two, long histories of exemptions to property taxes written into law--the Church, city-dwellers holding land in the city hinterlands, etc. (The latter being a larger problem in more heavily urbanized Italy than France, to be sure, but still a cause for some concern in parts of France like the Ile-de-France--which lay in the Pays de Grand Gabelle.)

So there were gabelle in the sense of taxes on not-hard-property in all sorts of ways--a customs tariff could be considered a gabelle, the salt tax, Florence had a tax specifically on drinkable wine. Like today, sales taxes in the Middle Ages were more popular than property taxes among the rich and powerful because poor people shouldered a heavily burden propertionally. A wealthy count might complain bitterly about a couple extra pennies spent but won't actually notice the difference. That same increase could break a peasant family. And unlike wine (Italy) or tobacco (France, later on), salt was a product that everyone had to purchase.

But massive wars or other sudden financial emergencies meant governments had to come up with new ways to get money, fast. In Italy, city governments often turned to forced loans. These fell disproportionately/entirely on rich people, who had the money to make loans. However, in these cases the governments at least promised to pay back the loans, and offered guarantees therefor, with interest.

The requirement to purchase a minimum amount of salt and laws regulating its use in impossible ways are two more ingenious engines for revenue generation.

First, a required amount of salt to purchase was an important feature of government income through a gabelle because it meant an expected amount of basic revenue to count on.

Second, the sel du devoir wasn't quite that curing meats was illegal; it was that to cure meats and such you had to buy more salt over the minimum (still taxed). Among other things, this basically meant paying fines for breaking the law--also a time-honored medieval revenue engine. From the Crusades on, the papacy leveraged bans on trading goods of war to Muslims, namely, timber and iron. Church officials knew perfectly well that the Genoese and Venetians were going to go right on trading said timber and iron to Egyptians and North Africans, and were content to collects "fines" as basically substitutes for taxes or licensing fees.

Pierre Laszlo, I think, has a good example of how these laws were basically meant to collect income in the breach. He cites a 1710 French case of salt smuggling/contraband trading that turned into a riot, in which government officials essentially busted an entire town. After customs officials confiscated people's property for breaking the law--extra taxation by way of fines--villagers resisted, rioted, assaulted some of the officials. The government charged and convicted 30 people. Despite the crowd's assault on government customs officials, still 28 of the sentences essentially amounted to heavy, heavy fines (seizure of property, up to 300 livres, etc). (Laszlo notes that the remaining two sentences, execution and life imprisonment at hard labor, were setting an example to people not to resist government collection of taxes/fines for evading the law.

And in that light, I'll add one more reason the gabelle on salt remained popular with governments: how much extra space was there along the way for corruption in the system? Consumers, merchants, and customs officials alike all got into the game. Just, as seen above, customs officials also had the power to crack down on end purchasers--an additional level of personal corruption or generator of government income, and generally at the expense of peasants.

Most scholarship on the medieval gabelle is on Italy, not France, but there is generally some comparative discussion. There's C.M. de La Ronciere, "Indirect Taxes or "Gabelles" at Florence in the Fourteenth Century," which gets cited in most economic histories of late medieval Florence, and the work of David Herlihy, briefly in Tuscans and their Families which he co-wrote with Christine Klapisch-Zuper and is an EXCELLENT read, or more extensively in "Direct and Indirect Taxation in Tuscan Urban Finance, 1200-1400."