BA serve Brussels from Edinburgh, if that's what you were asking, although as far as I know GLA has no direct link. In answer to your question, it's probably just that it's not a particularly profitable route- maybe not many people in that area need to go to CDG/BRU! Airlines are unlikely to link two cities just because they're 'big', there needs to be a viable commercial incentive for doing so, ie. demand from the market. It is more likely however that with FR down the road in Prestwick any service would face fierce competition from Ryanair anyway, which would probably make it very hard to make any money at all on the route
)
Your question about Ryanair is even more difficult to answer, as each has a specific market it appeals to, ie. FR will appeal to the backpacking tourist and the cost-concious/sensible businessman, whereas BA might appeal more to those who crave convenience and expect the service to be superiour in quality and more reliable. Different people will go to different airlines, though I admit there is a drift towards the budget carriers.
Hope that helps (though I doubt it!)
Cheers
Landing_24R