Monte,
In the beginning......there were no caliber restrictions to F-Class due to the failure of the original developers to think ahead at all. George Farquharson just wanted to shoot his TR (Palma) rifle with a scope and rest since he and his friends were getting a little long in the tooth. No one dreamed it would grow like is has less than 20 years.
The TR/Palma competitions then, as now were restricted to .308 and .223 due to the military history of the sport. A large part of the restriction had to do with the availability at the time of military ammo at low cost. Just as back in the day you had to shoot issue ammo in our 'leg matches' here in the States. The thought was it evened the playing field but that was not the case in my opinion.
In 1999/2000 the Canadian FC rules were wide open, "caliber less then 8mm; trigger weight more than 1/2 kg (1.1 lbs); rail guns and muzzle brakes are not allowed. Bipods and/or sand bags (fore and aft are permitted). Any sights are acceptable". Front rests were treated as sand bags.
The Rocky Mountain Palma Matches (RMPM) were one of the first in the USA to allow FC and their match program read "Any rifle, .33 caliber or less, any sights and/or rests, are permitted (sand bags, bipod) prone or back position. No muzzle brakes allowed." In effect, shoot what you had any way you wanted.
Over time some people became dissatisfied getting beat by people with other calibers and heavier rifles and at a 2000 meeting in Canada a weight limit was put on FC rifles of 10kg/22 lbs. I wanted it to be 16.5lbs so any "light rifle" BR would make it and the FC rifles could be used there also. I was out voted.
Then here in the States people complained about the same thing when they, shooting off bipods were beaten by shooters using rests. Discussions were held on weight limits, bipods, calibers, etc. Many want to allow calibers other than .308 and .223, but it was decided that a limit had to be set at some point and since other countries were setting up classes for .308 and .223 based on their shooting history we should do the same.
The reasoning being why have two OPEN classes. FO for experimenters and F/TR for diehards

The 8.25kg/18.18 lb weight limits was based on Palma rifles at the time with added scope and bipod. The first FCN's in 2004 used these rules. The lack of bullet weight was I think due to the USA's love of things bigger and faster. After all our NMC/LR history has no bullet weight restrictions. Only the Palma game does.
When the International Confederation of Fullbore Rifle Associations (ICFRA) got in the game and wanted to setup worldwide rules for F/TR they adopted the USA rules more or less. That in a nut shell is how we got where we are.
Larry