Why are gas thermometers more sensitive than liquid thermometers?
Gas thermometers are more sensitive because gas itself is, on average, more sensitive to changes in energy than liquids. That is to say, the same amount of energy will cause a greater amount of temperature change in a gas than in a liquid of the same substance. This is largely due to the nature of temperature itself.
Temperature isn't a "real" property, and neither is heat. It's simply the name we've given to our measurement of average kinetic energy of matter. All matter has a temperature, whether we're aware of it consciously or not, because all matter is moving to some degree; not even solids are able to exist with zero molecular movement except in theory. Heat is an even more specific and imprecise term because it's inherently comparative.
Gases are characterized by the fact that their kinetic energy is so high that any intermolecular forces between them are basically irrelevant. This means they are capable of responding to changes in their energy with a much more proportional change in their motion than a liquid or a solid, which have to devote larger portions of that energy to disrupting intermolecular bonds.