I got the impression from the first Chronicles that the only reason Covenant could be transported to the Land in the first place was because both Foul and the Creator chose him, otherwise the summoning would not have worked. After someone has been summoned once, though, it seems they can be summoned as many times as needed after that...hence why Linden's "accidental" summoning together with Covenant in TWL in enough to allow her to be summoned again in Runes (and take Roger, Jeremiah, and Joan with her). Foul simply can't choose whoever he wants. (And the Creator is unlikely to say "Ok, Foul, it doesn't seem fair to keep sending you the guy who's able to beat you, I'll let you pick someone else")What restricted him then from reaching out to some lame idiot that didn't have the passion for The Land that Covenant learned in the First Chronicles, but who held a white gold ring, and manipulating him? Covenant, despite the Despisers insistence, is not so easy to manipulate.
Of course, if that explanation doesn't cut it for you, I have an alternative. I think we all understand the reasons Covenant was chosen the first time around. The reason Foul wanted Covenant again for the second Chronicles would be simply that it would be difficult for anyone other than Covenant (who has seen the Land at its most beautiful) to care enough to fight for the Sunbane-stricken wasteland that it had become in the second Chronicles. After Covenant dies, of course, Foul doesn't really have a choice in whether he stays...I could speculate on why he wants Linden in the Land for the last Chronicles (other than my theory that the only way he could even bring anyone else into the Land is on the heels of someone who had already been summoned before), but until we really know Foul's plan, it won't mean much.
As to why he can't use Joan...Linden's theory is that it's because she's too insane to be manipulated. Also, it could be that her insanity prevents her from being anything greater than Foul's tool, and it was mentioned in the first Chronicles that a tool is neither capable of threatening nor protecting the Arch.