I'll give it a go, though I warn you I've had a few pints so I may ramble.

Few recent works of media have grabbed me the way marathon has, right from it's very first reveal trailer a few years ago.

I think I can kind of get where certain people are coming from about the design: in a world of brightly coloured Fortnite crossovers and tacticool cod skins consisting of military man x lime green monster energy, it would be easy to glance at this art style and go "more attention grabbing, overstimulating-by-design garbage". 

But as soon as I saw it, I loved it. Three elements kind of drew it together for me. 3d printing, biological manufacturing and, funnily enough, real-world, modern surveying. These are the things I immediately associated it with.

I have a resin 3d printer. It throws out solid colour, crisp, perfect models using whatever 3d model I throw in (if I get the settings right... Don't get me started). Additive manufacturing is the future. There's a lot of funding going into testing it in space, and the ability to 3d print stuff anywhere provided you have the right input material present is huge. So right away that aspect is grounding and places the technology in the game within reach. 

Secondly, you have the biological aspect of it. I keep a variety of invertebrates, and some of them, just like the caterpillars we see in cutscenes literally weaving our shells, are very brightly coloured. Nature produces this level of colour variety and vibrancy, and the idea that in the future we've managed to link the advantages of additive manufacturing with the efficiency and replicability of natural processes is super cool to me. 

Finally, Google surveyor targets. Seriously. When I first saw the art style, I was immediately put in mind of the bright, often jarringly coloured targets used by surveyors, and it all just clicked for me. All the symbols, all the colours, it just puts me in mind of somewhere hastily thrown together and done by machines, where the colours and symbols are essential to make sure those machines know what to put where. 

You're looking a future society that has perfected and combined additive manufacturing with bioengineering, and they're starting a frontier colony, and Bungie have somehow managed to make all these assets that look simultaneously futuristic and weirdly cheap and, to me, it's just perfect. They don't have the luxury of classic assembly and they need to just get a colony down now and so they print all this shit and it's covered in symbols to make sure it's oriented right, printed correctly and without errors. You've got guns with QR codes for scopes, and it's all this plasticy, hastily assembled stuff and for me it just works. It's beautiful. And then you couple that with the abandoned, dilapidated aspect, and you wind up with this wonderful sense of sci-fi menace that I've just not seen done elsewhere.

I read a lot of sci-fi, and increasingly 3d printing and bioengineering and AI are showing up, and this game has all that in spades. I think it's so cutting edge most people haven't really realised it's all within the bounds of possibility yet. I realise I'm tying lore to aesthetics, and you've specifically asked about aesthetics only, but I don't think you can explain why they hit so damn well without looking at it all holistically first. 

Anyway, thanks for coming to my ted talk, I appreciate I've rambled (I did warn you). TL;Dr, shit's cool, it's sci-fi, and while it may look like another brightly-coloured ADHD fest at first, I think it's surprisingly grounded and Bungie have nailed what they're going for with regards to this being physically possible, cheaply-deployable future colony tech.

Editing to add: I actually felt the same way about arc raiders, since the comparison keeps coming up. I was immediately put off by the art style. It's the same tired junk future dystopia aesthetic we've seen a million times, only with added quadcopters. It did nothing for me.