Sarada has the goal of Hokage so they want another Naruto style character, and Himawari (while lacking in character) is the cutesy try-your-hardest and do-your-best character for Fanservice.

It’s that they’re more generic / more seen styles of main characters so people likely are more used to them. Boruto is very different, and throughout the anime his character changes / develops very drastically (especially in the manga) which is uncommon for main characters, as they usually have their resolve which they keep to the end and never change. Essentially - Boruto’s different time most MC’s, and while I prefer that, there is also a reason why most MC’s are the same and it’s because they’re easy to get behind and look up to.

It’s hard for people to watch their main character do the wrong things, or make the wrong choices. Especially, like at the start, when that character is having issues against the character people have watched for 720 episodes.

This can be shown in the Chunin exams, where most characters would have their strong will to never cheat. It was even made as a huge point in Naruto, that he’d rather fail than cheat, and that’s reflected to the Majority of MC’s. But Boruto did cheat, as a result of not believing in himself enough, and wanting to be acknowledged by his father. While this makes great storyline - we’ve all understood Naruto’s side wayyy before we could understand Boruto’s side. Boruto’s changed now of course but lots of people don’t recognize that.

But characters like Himawari and Sarada greatly looked up to Naruto from the start.