Summertime Saga just has a broader appeal with it being more vanilla and it's lighter tone.
The Summertime Saga town feels basically like a regular town whose women just happen to be really really horny and it serves the fantasy of the kindof shy, friendly, nerdy kid solving all the problems and getting with tons of women in the process.
While Taffy Tales's town is much darker with everyone having some dark secret. Drug addiction, drug dealing, brainwashing, sex traficking, blackmail, rape and so on and so on.
I am not saying TT needs to become lighter, that would be dumb as it has an established audience already, but it's not hard to see that Summertime Saga just has more broad appeal.
I personally like both of them for different reasons and I think it's good that these two games with fairly similar sandbox approach go in different directions in terms of appeal instead of trying to copy one another.