1. The Mefarshim understand that the form of the Heichal should not be changed from that which was commanded to Moshe Rabeinu. Therefore, all the Shulchanos should be in the north, because this is where the Torah commanded us to place the Shulchan of Moshe.

2. This is implied by the words of Rabeinu Bachye to Shemos 25:1. He explains that one should not think that Hash-m commanded that one Shulchan be made, and Shlomo ha'Melech instead made ten. If he would have done this, he would have trangressed the prohibition of Bal Tosif. Rather, Hash-m told Moshe Rabeinu on Har Sinai that when Klal Yisrael will build the Beis ha'Mikdash, they will make ten Shulchanos. Accordingly, the Jewish people had a Kabalah, a tradition, that there would be one Shulchan until the time came to build the Beis ha'Mikdash, at which time there would be ten. Shlomo ha'Melech acted in accordance with the Kabalah.

Since the construction of the first Shulchan and the construction of the ten Shulchanos were commanded by Hash-m at the same time, it is easy to understand why the other ten Shulchanos should be built in the north in the same way that the Torah specifically commands that the Shulchan of Moshe Rabeinu be in the north.

3. Rav Chaim Soloveitchik zt'l also explains (in Chidushei ha'Shas, section on Kodshim, "The Din of the Shulchan") that the command to make additional Shulchanos is included in the principle of "ha'Kol bi'Kesav mi'Yad Hash-m Alayi Hiskil" (Divrei ha'Yamim I 28:19). This principle teaches that the Beis ha'Mikdash may be built only according to the guidelines that David ha'Melech recived from the Nevi'im, and one may not add to or detract from those guidelines. Rav Chaim says that the ten Shulchanos presumably were also made the same way in which Moshe Rabeinu made them.

According to this approach, it is logical that the ten Shulchanos were in the north, similar to Moshe Rabeinu's Shulchan, which was in the north.

Kol Tuv,

Dovid Bloom

Here is a slight addition to the above reply, expounding somewhat on the idea behind it.

The reason why the Gemara learns the Shulchanos of Shlomo ha'Melech from the Shulchanos of Moshe Rabeinu is that any Mitzvah given by the Torah is given for all generations. Thus, when the Torah gave the Mitzvah to Moshe Rabeinu to place the Shulchan in the north side, it gave that Mitzvah to place the Shulchanos on the north side for always, whether in the Mishkan or later in the Beis ha'Mikdash. The Mitzvos of the Torah were not given only to Moshe Rabeinu. They were intended to be binding forever.

Kol Tuv,

Dovid Bloom