Silicone oil is preferred because of its:

Excellent Insulating Properties: High dielectric strength ensures effective insulation.
Thermal Stability: Performs reliably over a wide temperature range.
Fire Resistance: High flash point and low flammability enhance safety.
Chemical Inertness: Resistant to oxidation and chemical reactions, leading to a longer service life.
Environmental Friendliness: Generally non-toxic and less harmful to the environment.
Low Maintenance Requirements: Reduced need for frequent oil changes and maintenance interventions.
These characteristics contribute to improved transformer performance, safety, and longevity.