Well its because the battery inside of the gamepack that allows you to save the game data is dead now or its a bad gamepack then.
Can your Pokemon game still hold a save?
If your Game Boy game is no longer able to hold a save, yet can still be played fine, that most likely means that the battery in the game cartridge has run out of energy.
You can easily replace this battery, restoring the cartridge's ability to store save data. This replacement can be performed for much less than the cost of buying a new game. You just need to purchase a new battery.
**Basic soldering skills required**
New 3 Volt Coin Cell Battery - CR2025 (165mAh) or CR2032 (220mAh)
Fine tipped hemostat or needle-nosed pliers
Flip over your cartridge.
Using a hemostat, needle-nosed pliers, or anything else that you can grip it with, unscrew this little screw by turning counter-clockwise.
Once the screw has been removed, turn cartridge over, slide the front cover downwards, and pull it off.
Now you can see the circuit board, and the battery that allows one of the chips to retain the game save data.
You can see that both Pokemon Red and Silver cartridges look very similar, except that the Silver circuit board has a crystal oscillator in the upper left-hand corner to control the passing of time in the game.
Coming next: Removal and replacement of the battery