1) The show in certain respects is structured the same as TNG and VOY, so viewers were burnt out on it. This was happening at the same time Galactica and Firefly were doing relatively new things with serialised storytelling. It just seemed old-hat and uncool. Now that we're a couple of decades on from ENT, it is closer in time to TNG and VOY than we are to ENT, so it gets evaluated on a more level playing field. Objectively speaking, I feel TNG still beats ENT, but personally, I think it compares very favourably with VOY. I have no issue with heavily serialised stories, but it seems like only a few TV writers are capable of doing them successfully, for whatever reason. The pendulum is swinging back the other way a little bit now, and standalone shows such as SNW and early Mandolorian are getting good feedback. Similarly, I can put on an Enterprise episode, and I don't need to make a time commitment for a season long arc.
2) Visually, it's aging very well. It's subjective, but for my money, it's the last series that had a visual language that is mostly its own. Kelvin films look like Apple stores and early Marvel films. Discovery just looks like Mass Effect to me. Picard looks like a hybrid of Mass Effect and 90s Trek. SNW looks amazing, but is deriving its visuals from TOS and other mid 20th Centuray American influences. ENT's production design stands up today.
3) Maybe there's a self selection bias here? Maybe the people who come around to the series are looking back with a bit of nostalgia, and remember fondly shows that were made in a slightly retro way. I don't want to become a grumpy old man, but I miss watching characters that I can personally identify with, and use fewer colloquialisms. I have a hard time thinking someone twenty years younger than me would be likely to get into the show. Time moves on, and what people want to watch changes but I still like, and am nostalgic for, shows from the 80s, 90s and early 00s as well.
4) Enterprise is just fun. It's an engaging adventure show, with all sorts of plot lines, from T'Pol telling offbeat tales about Carbon Creek, to the Enterprise getting trashed in Azati Prime. There's a lot there to like.