The earliest fiction didn't posit a human-only universe, and Ardan Sortek not only describes the Stein's Folly "pinks" as sapient, but notes that humanity has encountered several other sapient species in its explorations.
That being said, later developers firmly declared that (Tetatae notwithstanding), there are no sentient aliens in BattleTech. It would have required a whole new set of rules mechanics to cover alien tech, potentially making game balance a nightmare. In addition, there'd be a tendency to have the human factions banding together against the alien threat, decreasing the factional diversity that is at the core of BattleTech politics.
At the 1990s developer meetings, every now and then somebody would suggest introducing aliens, or bringing back the Wolverines. Finally, someone quipped that the Wolverines were off in the deep periphery fighting to protect the Inner Sphere from the aliens, so we'd not be seeing either for the foreseeable future. That's pretty much been policy ever since.