Could still be because it is the place where all races can mingle withoutthe locals raising eyebrows. We don't see any Githyanki in the lower city.
To fuel your ponderings a little further: If you snoop around the Devil's Den a little you can find three documents White knight, bishop and pawn orders, that talk about canceling the debt of three of Raph's debtors in exchange for their help in bringing about the promotion of Flame Portyr (the Flaming Fist who is currently bespelled by Ethel). The name Portyr shows up again in the basement of the Stormshore Tabernacle, in a letter in which a Duke Portyr wants a transcript of all prayers offered at the tabernacle - the vicar refuses of course. So my conspiracy theory is, that Raph wants to know what the citizens of BG are praying for, so he can prey on the desperate and in exchange helps to promote the Duke's relative.
