Why did Nintendo's patent on FC expire in 2003?

Patents have a protection period, and the protection period of invention patents in China is 20 years, so everyone can use this patented technology for free. An important purpose of the existence of the patent system is to make the new achievements of mankind public and finally make them available to more people. Of course, there are many reasons for the failure of the lawsuit you mentioned, not necessarily the expiration of the patent protection period, but the specific circumstances of the case.