As a relatively new fan (I only really got into baseball in 2019) I feel like my perspective is probably pretty different from most here.
I would say that the main reason I'm not watching baseball on a regular basis is that it's too difficult/expensive to do so. If there was some way that I could casually switch on a game without thinking about it, I would probably do it when I don't have anything else going on on a given evening.
I have watched two Twins games this year. I watched one in a bar, and the other was at Target Field. I have caught partial games at bars and restaurants.
I don't have cable (or even regular broadcast TV) and don't want to pay for cable. I've seen some fans on here say "just spring for MLB.tv," and that's a perfectly fine solution if you've been watching baseball for years and it's worth it to you, but no casual fan or newcomer is going to pay that subscription fee (or get cable) simply because they're curious about baseball.
Ultimately, you just can't get young people interested in something that they need to go behind a paywall to even get a taste of. There's too much free entertainment out there. People in their twenties would rather watch Youtube than subscribe to Netflix. There is currently no equivalent to just turning on any old TV and tuning into the game. You need to choose to pay for baseball before you can figure out if you like baseball. There's no entry point for viewers who are just mildly curious (except for a few free games on Youtube which allow you to watch the home team just a few times a year).
I dragged my teenage foster daughter to a Saints game earlier this summer, and after complaining the whole week before, and complaining the whole way there about how baseball is boring, she loved it. When she saw it, it clicked for her. She was absolutely captivated by it. The sport has not lost its ability to enchant young people. If you put it in front of them, they'll still get invested in it, it's just not conveniently available.
Many of you have lost some interest due to changes in the game - The shift, three-true-outcomes hitting, longer games, etc. While I don't doubt that games may have been more interesting before these changes took place, I had no problem getting into the game during the Bomba Squad era. (in fact, the first player who I got excited to root for was Miguel Sano). I certainly wouldn't mind a faster-paced version of the game, but the current game has been perfectly sufficient for me to enjoy it, even as a social-media-addicted millennial. I'm also not losing interest now that the Twins are having a rough year and have a rough path ahead.
Currently I follow games on Twitter, read recaps on this blog, and occasionally tune in on the radio when I remember to do it. I've been to six Saints games this year, and will probably be able to make two or three more (if I'm going to watch prospects, I might as well go where I can afford good seats).
I don't want to pay for a subscription if I don't know how much I'm going to use it. Based on my personality (and I can't speak for all younger/casual fans here), the best way for MLB to get me to pay to watch baseball would be to provide a way to watch individual games for the price of a movie rental instead of requiring a subscription. But this would not appeal to me if I wasn't already following baseball. In order to hook new viewers, they need to bite the bullet and make regular (not necessarily all) games available for all teams for free (with commercials as needed) on a platform people already use. Which shouldn't be impossible, since they used to do it on TV and it doesn't seem like they went bankrupt then.