Well... Actually there may be thousands of interstellar subspace connections in every system. As far as I remember, techroom says there are many nodes in Sol but Sol-DS node was first stable enough to travel through it so it's the only listed one. Subspace node network may be much dense than official map describes. Plus we have numerous listed and non checked nodes in various systems where Shivans attacked in FS1 like Ross 128, Ikeya, Ribos etc. Sol may be connected with much more stars, but GTA never discovered more stable nodes.