I think an issue is a sub can only have 2 megathreads at once and there are several types of posts on the sub where people have similar feelings, like photos, weather, crime, where they rather those are just kept in a megathread instead of taking up the front page. My guess as to why Reddit limits to 2 megathreads is they know megathreads can help reduce the amount of activity on the website. Having the same comments scattered across many separate threads and then those posts / threads fading from a sub's front page within a day allowing a repeat of the same thing means a lot more activity on this website, which looks good for their stats and helps get them more VC and advertiser money.