Five reasons for me (lol I started with two):
1. Novelty is definitely one. Not only was it a new feature in NL that was hugely expanded in ACNH, but it was some I've dreamed of since the first game. The fact that there were items that were literally designed to be placed outside... that then couldn't be placed outside... drove me nuts! So I was PUMPED to finally get the chance to decorate outside and you bet I was going to 100% take advantage of it.
2. There's just more versatility and space outside. Don't get me wrong, I still like interior design. But I feel like there's so much more potential with outdoor design. The island is so big compared to tiny houses, and I have a lot more freedom to get creative. If I want a fun beach, I can do it. A playground? Space for that, too! An orchard and farm and forest and campsite? Space for all of it! Whereas, inside it's like... ooh, I want a library! Oh wait. What room will I need to get rid of to accommodate that?
3. More people can see the outdoor space. I mean, I love how my house looks, but the vast majority of people who visit my island will never see it because they're there to drop or pick stuff off/up. They're not going to bother going to my house to analyze the interior design (and... I'd be a bit weirded out if they did

). But, with exterior design, I still get to enjoy hearing "wow, your island is so nice!" when they leave
4. I've always been the type to decorate my interior until it's done and then... just leave it (even if the HHA apparently frowns on that

). I occasionally change little things, but only with my main character. For my side characters I have goals of how their house will look and, when it's done, it's done. But I've taken a different attitude towards my island, changing it up fairly often (AT LEAST seasonally, if not for each holiday). It keeps the game fresh and gives me new goals! Like, I spent a ton of time on my Fall/Harvest Festival. I LOVED it. Then I updated my DA so everyone could see it... and took it all down to replace with a Winter/Toy Day theme, which is currently a WIP!
5. And, lastly, though I touched on this in other points- ACNH has a really well-developed system put in place to design a town down to the tiniest detail. I mean, I enjoyed the original game and previous games. It was fun exploring the town and meeting neighbors. But I never felt a deep connection to the town because I had little to no control over anything. I relished the tiny bit of control ACNL gave me, but ACNH stepped it up big time. And that alone made me adore the game. Sure, it didn't ship with all the holidays. And it lacked the fun mini-games from ACNL. And villager dialogue needs work. But I could control basically every detail on my island, things I'd been longing to do for so long (like, I can't count how many times a villager moved into the worst possible spot in ACNL and drove me nuts!), and I want to take full advantage of it. Sometimes there are things you wish for for so long, then you get them, and it's like... eh. Why did I want this, again? BUT that is not even close to what happened with ACNH! The details I'd been dreaming of that they added are EXACTLY as cool as I'd hoped, and I love using them!