Recently indies/exdevelopers of the franchises have successful launched an alternative to key franchises that were famous in the past and these have been widely sucessful, however I think Sims is the only one that someone hasn't been able to capitalize on, and why is that?

Recent Examples:
City Skylines is much better recent Sim City
Planet Coaster is much better than recent Rollar Coaster Tycoon
Stardew Valley is much better than recent Harvest Moon

Etc. 

Hard to say, maybe Animal Crossing is enough for people, and Fire Emblem for the marriage fond.

Tomodachi Life is pretty popular.

Also - there's loads of life simulation games targeted to the casual crowd on tablets/smartphones. We gamers generally ignore them though.

And there's also Second Life.

Farm simulators are pretty much life simulators set in a rural context.

Sims 2 nailed it, there simply isn't any need for a new game in this style. Even Sims 3 and Sims 4 were uneccessary. There are games from the past that can use somewhat of a makeover, but sometimes there simply is no way to beat the original. Just like with Transport Tycoon. Transport Fever tried to do everything better than the Original but failed. It would be the exact same thing with Sims 2. You just can't beat it. So why bother?

Oh gawd yes, the sims defly needs a better alternative. I've been wondering and asking abt this too on steam forum.

Sims3 concept was great. But instead of optimizing the game, EA dumbed down the game. The world, texture editing, story progression are removed from sims 4.

If there's a developer decide to make a new game for this genre, i really hope they dont handle dlcs like EA does

Agree with you. The Sims 2 improved on the original. 3 and 4 never capture the same magic. If EA cannot make a new one in the same level, don't think anyone will. Neither need too.

I think Sims is two folds game, it is one managing your many sims, and one part building the right houses. I don't feel the farming sims solve either of them really well, and are more focusing on growing and paying off wealth as well focused on creating relationship from main character to others. 

I'm still waiting for someone to create a Sims spiritual successor. The Sims 4 was such a step-down and feels so bland compared to the others that I never even bought it, and that's coming from someone who loved the previous games. Even the expansion packs look boring.

This