Why a spin off when a sequel can do?

1) It's easier for the writer/s as the show will start from a clean slate and not be bound by continuity from the first season

2) Getting the same casts from a 5-6 year old drama is very hard as they will need them to clear up their schedules at the same time for filming

3) The first season pretty much wrapped everything up in the final episode and the main couple already got together. Shows where the main couple are already together tend to get less viewership

Because they thought the premise deserves to have a more socially relevant plot, with decent actors from leads to veterans. 

The writer created a world where the women of a certain family line have great strength. The writer wants to continue building this world and explore other stories in that world, in this case, the other branch of the family.

In this new story, the writer revealed that there are certain women born with extra abilities, like jumping high and amazing speed.

It is also an opportunity for the writer to improve their craft. Especially in Korea where sequels are not a thing. The writer is probably thinking, "Can we pull this off?" The director probably thought, "Can we do this?"

 Surya Gentilhomme:

Can’t live up to the original when making a sequel for this one. A spin off is a commercially safe bet and there is lot of room to improvise. And it’s hard to compare it to original since actors changed too. 

A spin-off is not expected to live up to its parent. It's a whole different story. The only thing that connects it to the original is its shared history. We call this in creative writing as “shared world” or “shared universe”. It is frowned upon to compare it.

Think of it this way. You have a farmer. You have a land. Not only that, but you call this land “The Soon World”. In that land, you have apple and orange trees. Do you compare apples and oranges just because they were planted on the same land called “The Soon World”?

Of course not. Apples and oranges are two different fruits, even if they share the same land property.

In legal terms, it is called a “derivative” or “derivative work”. For example, there are many derivative works of H.G. Wells's "The War of the Worlds". In fact, there is a thriving community of works that co-exist in "The War of the Worlds" shared universe wherein different stories from around the world were told. You will not see comparisons of these works, because again, in the creative writing space, this is frowned upon. You don't compare derivative works, or "spin-off" in TV/film terminology, even if it was written by the same author.

Second point. You said it yourself, "it's hard to compare it to the original since actors changed". That's the second reason why you shouldn't compare at all. Just like in the thriving community of H.G. Wells's "The War of the Worlds", how can you compare a story set in the same universe but different country to the original by H.G. Wells set in a different country? You just don't and can't. Even if they share the same universe, even if one or all of the characters appeared in each other's work (called "cameo" in TV/film terminology), they are still apples and oranges.

Going back to the Strong Soon franchise, if they hired the same actors but for different characters, just to allow for comparisons, well, that is called a reboot, not a sequel, not a spin-off, as far as TV/film is concerned.

Strong Girl is not a reboot. It is a derivative work based on Strong Woman. In fact, the Strong Girl branch of the family was mentioned in Strong Woman, thus making it CANON. Firmly establishing that Strong Girl is not a reboot, rather it is the story of that other branch of the family. (Which was, quite frankly, highly requested by fans in the first place.)

Here's another way of looking at it. Some relative of yours started to write your life's story. Later, this same relative wrote the life's story of your cousin. Do you compare your life story with your cousin's life story?

You don't and shouldn't. Your story is yours. Your cousin's story is theirs. In the same way that Strong Woman and Strong Girl are two stories that shouldn't be compared.

Strictly speaking, Strong Girl is not Season 02 or a sequel of Strong Woman. But, MDL is MDL, they love to confuse the public and won't accept any corrections. *shrugs*

It would be rude to say that but kdrama writers don't have the brain to write a proper sequel. I have seen a lot of my favourite drama are ruined because of this.