re: Why are mice always the subject to experiments?

As other have stated mice are relatively cheap to house, keep, and feed. They are mammalian so a lot of the way their body and systems functions still apply to us so their use in comparative medicine research is high. That isnt to say that non-mammalian species cant serve as excellent research models but are often more limited to specific scopes.

Mice have a vast history behind them in regards to research to dig back on and have as background data to build on. Large companies exist with hundreds of specific strains of mice like Jackson laboratories and Charles River.

Animal research must follow the 3R mantra (reduce, refine, replace). Part of that is using the lowest phylogenic species suitable for the research which often falls in the mouse's lap.

If you really are interested in this you can read "The Guide for the Care and Use of Laboratory Animals in Research" online for free and goes into a lot of detail.