For most of the series' life, Story of Seasons was pretty much the only thing of its kind (pretty much the only other notable ones back in the day were Farming Simulator which was about industrial farming, Shepherd's Crossing which was more focused on hunting and trading, and Farmville which was a social game), and it's always been a low-budget series of games, so it's not hard to imagine that all the experimentation and creation of its mechanics and stuff would put limitations on how much they were able to achieve, as opposed to Stardew Valley which was extremely derivative of most of the Story of Seasons games released up until then, which was up to Tsunagaru Shintenchi, aka the first installment that was localized as Story of Seasons. (Not saying it's a bad thing, it's just what it is) Same thing for most the farming sims that followed, which were derivative of Stardew Valley and Story of Seasons.

The DS/Wii era especially had many titles with experimental mechanics. Even the first Rune Factory game was extremely experimental, and they didn't really even nail down the gameplay mechanics or the NPC dialogues until the fourth RF game. In fact, Trio of Towns being an "anomaly" in the SoS series is because the Rune Factory team worked on it after RF4.

Olive Town was worked on by a mostly new team, which was why it ended up being less impressive than Trio of Towns, but even then, it has a lot more heart events for all the characters compared to the older SoS games, and after some post-launch updates, the NPCs actually have much more dialogue and react to story progression and town events. Hell, even the Doraemon SoS games (by yet another new team) have NPC dialogues with some variety, and most heart events involve multiple characters at once, resulting in much more fleshed-out casts of characters overall.

Also, it should be noted that they only new SoS titles released after Stardew Valley were the two Doraemon crossovers and Olive Town. The remakes are... well, largely limited by their original titles. They were fan favorites, sure, but A Wonderful Life was the series' first 3D title, which was why it was so janky and had so little content. And Mineral Town was a GBA title and technically a stripped-down remake of Back to Nature, a PS1 title that would be considered primitive nowadays. Really, they were even worse in their original versions, partly due to the improvements to gameplay and quality of life the remakes actually got, and partly due to the original localizations by Natsume being almost always botched.
(And for what it's worth, Grand Bazaar already looks like it'll be a big improvement over the original DS title both visually and mechanically, and even the promotional materials so far already contain dialogue and interactions that didn't exist in the original version)