I'm no web developer, but it's still very easy to see why.

So they can push you to download their app. Saidit mobile website works fantastically well, so does the redreader client. There was absolutely no reason for redbit to make a shitty website it other than to push you to into downloading the shitty reddit app. Now if you ask why the app is so shitty - because they wanted it to look like Insta, snapchat et al so people can find familiarity.

Why do they want you to install the app you ask? Well, isn't it easy! Reddit has had a hard time selling ads. Their main source of income is selling user data. Today the best vector to get user data is getting mouth breathers to download your shitty app.

They have also made a stupid redesigned desktop website - horrible stuff. It's like those generic websites with a vertical bar of worthless content in the middle of your screen and the rest of your space is wasted. I think that's also there for familiarity. Easy to get Instagram users to join your website.