The scientists are not really limiting themselves, IBWIW. just look at this Wiki about Titan:
Although all living things on Earth (including methanogens) use liquid water as a solvent, it is speculated that life on Titan might instead use a liquid hydrocarbon, such as methane or ethane. Water is a stronger solvent than methane. However, water is also more chemically reactive, and can break down large organic molecules through hydrolysis. A life-form whose solvent was a hydrocarbon would not face the risk of its biomolecules being destroyed in this way.
Edited by Abramelin, 07 February 2013 - 09:01 AM.