Generally, airlines do not like to be directed to particular airports. They want to go where thee traffic is, and if the traffic includes a lot of domestic passengers, that will be a major attraction. Attempts by national authorities to direct traffic to specific airports are often viewed with suspicion that a particular competitor may be getting an advantage. So traffic rules must be seen to be fair and apply equally to everybody.
Both Paris and London have had in the past an airports policy, ostensibly to separate traffic by the direction it is coming from and going to, in order to simplify the management of congested airspace. But in both cases ‘quality’ airlines tended to get directed to one airport and ‘the rest’ to the other one.