* Nail polish is made of polymers: These are long chains of molecules that form a hard, colored film on your nails.
* Solvents break down polymers: Acetone and ethyl acetate are good at dissolving these polymers, essentially breaking the chains apart and making the polish soft and easily removable.
"Spirit" is a general term that can refer to alcohol, especially grain alcohol. While alcohol can sometimes be used as a weaker solvent, it's not as effective at removing nail polish as acetone or ethyl acetate.
So, the reason "spirit" isn't the best choice for removing nail polish is because it's not a strong enough solvent to effectively break down the polymers in nail polish.