Hi @Dane@,
there is no particular reason for attaching the mainstay to the bowsprit and not to the stem. Both solutions, mounting to the stem and to the bowsprit, were commonly used (probably more often to the stem). The attaching to the stem is on the plans of Miller (based on drawings by Chapelle) and Chapman; the attachment to the bowsprit can be seen in the plans of Edson, Feldman or the Sloup "Mediator" (by Chapelle). I mainly use Feldman's work and the angle between the mast and the stay was probably the key aspect for the author. This angle cannot be too small, because one of the most important ropes on the ship would not fulfill its function. I have no engineering knowledge to calculate the forces acting on the stay and choose the optimal angle (:-) so I believe Feldman was right. Attaching the stay to the bowsprit was not unusual, so I found it possible and credible.
Below is the same drawing of a Bermuda Sloop (Chapman) with a different reconstruction of the sails (Chapelle and Edson).
Greetings
Tomek