Cecily's pretty low marriage and Elizabeth's foreign one were chosen to neutralize the Edwardian claim. Anne and Catherine can very well be married in the high nobility to Ricardian supporters. Anne was to be married to the heir of Norfolk so a match between Catherine and the heir of Northumberland made sense. Arundel has already his daughter married to Richard's heir presumptive and is also married to one of Elizabeth Woodville's sisters so better do not give him eventual ideas...But would they *stay* loyal?
Two centuries before, a Holy Roman Emperor observed wearily that "No Pope can be a Ghibelline." In the same way, would any son -in-law of Edward IV be able to remain a Ricardian?
Isn't marrying someone widely perceived to have a better claim to the throne than the present occupant almost bound to drive them apart, rather as elevation to Canterbury ruined relations between Becket and Henry II?
And if EoY is married to a foreigner, she might be viewed as no longer eligible, though such technicalities are easily be overlooked if someone wants to badly enough.
Edmund de la Pole, well married to Catherine of York he would likely be seen as rival to his elder brother and well marrying an officially illegitimate girl to the younger brother of his heir presumptive would be a bad idea. Edmund de la Pole need an heiress as bride