From a mechanistic point of view, the cyclization is pretty easy to understand; The isopropenyl group needs to get activated by either a protic acid (Brønsted-Lowry), or an electron-deficient atom center of a “bigger” reagent such as ZnBr2 (Lewis “acid”).
In the protic case, think of the proton as being added to the double bond and in principle, the proton can become attached to (CBD numbering) either C8 or C9, forming a positively charged carbocation at C9 or C8.
In the first case, the carbocation is said to be primary, and in the other case the carbocation formed is tertiary. Tertiary carbocations are much much more stable than primary carbocations and you will in practice only form the tertiary carbocation which now forms a chemical bond with one of the free electron pairs of either of the phenolic hydroxyls. D9-THC has been formed.
Since the ring closure is of an intramolecular nature, these two discrete events can virtually happen simultaneously in a pseudo-Sn2 fashion. The “lifetime” of the carbocation is very very short.
In the case of a Lewis acid, think of the metal center as co-ordinating to the double bond, the electron density of the double bond shifted towards the metal, and again, you have created (a little) positive nature at carbons 8 and 9, and again, 8 is more positive than 9, and ring closure occurs as above.
From this description, one can draw the conclusion that you don’t really need a fully developed positive charge for the cyclization to occur.
Protic acids are of type HX, carboxylic acids, sulfonic acids, phosphoric acids, etc. Anything that can provide a proton, H+. HX is often unnecessarily strong and there can be side reactions. Organic synthesis is very much about tuning the combined substrate/reagent/catalyst reactivities so you create what you want, while suppressing competing reaction pathways.
Sulfonic acids just happen to be very versatile and easy to handle protic acids. Now, why pTSA?
This is where it gets interesting and very few chemists know the backstory. It is because of an artificial sweetener, saccharine, that pTSA found itself on the throne of protic acids, and it all started towards the end of the 19th Century, when Organic Chemistry was still lying in its cradle.
Can you figure it out?