Quick Thoughts on the Origins of the Theatrum Machinarum

1. Books were a way of gaining patronage and demonstrating competence. Since there was no school credential or professional accreditation, engineers had to demonstrate what they knew in a novel way. Perhaps the TM were as much portfolio as anything else. Political leaders may have struggled with a crisis of information i.e., there wasn't enough.
2. Math was important. The mechanical arts could be viewed as either manual and below dignity. Or they could be viewed as an important part of mathematics. The introduction was thus very important.
3. Nobles loved machines. As demonstrated by Wolfe, everyone was fascinated by machines because the demonstrated many of the noble qualities of the courtier. Ramelli and Besson's machines also appealed to an emergent sense of utility.
4. Machines were safe. Competitive professionals such as architects were hobbled by their interactions with self-interested guilds. Masons, for example, were very invested in protecting their own knowledge. Ramelli and Besson's machines were not the domain of any guild. Sure, carpenters and masons built machines (as demonstrated by Plate 43 of Villard de Honnecourt's manuscript), but none of them had specific domain over their design and creation. Instead, machines were individually the domain of artifice.
5. Fortification design was... well, I just don't know. Besson didn't create a work of fortifications because he wasn't an architect or engineer. Ramelli's designs may have been stolen by Bachot. Errard did create a work on fortifications. Perhaps there was an issue with Vitruvius. Machines were mentioned in Vitruvius and became increasingly important with the translation of Hero, Archimedes, the Mechanica, etc. But maybe architecture was completely the domain of Vitruvius and from a humanistic perspective everything had already been said.