Why don't ground-types (fighting) in Pokemon TCG have resistance to electric?
Pokémon Trading Card Game follows original type effectiveness table only loosely. In each generation, there can be found some patterns, like originally Ground Pokémon having Lightning Resistance in earlier generations, but those usually change in time and also some cards can diverge. Some of those patterns were applied according to one of the types of dual-types Pokémon, without regard to the second type.

Let's have Gligar as an example. In the past, there was a pattern of Originally Ground-type Pokémon having Grass Weakness¹. Even though Gligar is damaged normally by Grass and is resistant to Bug and Poison² (types that were represented by TCG Grass type back then), it was printed with such Weakness:



In the past, there was also a pattern of Originally Flying-type Pokémon having Lightning Weakness³. Even though Gligar is immune to Lightning² in games, it was printed with such Weakness:



Later, Gligar kept its Water Weakness and Lightning Resistance for a long time, loosing the Resistance during 6th Generation, which is the same for Sandshrew as well. It's possible that card creators changed their patterns recently. You can compare Gligar from 2012 (5th generation) with Gligar from 2014 (6th generation):



During the 7th generation, Gligar's Weakness changed to Grass once again:



Officially, there is no information about how the conversion from game types to TCG Weaknesses and Resistances work.