What particular features of Fifth Edition make it a big tent game in a way that other games are not?
The D&D name & history.
It's more a bartizan than a tent, though - it's the gateway to the hobby.
There is the (apocryphal and untrue) quote attributed to Pauline Kael, along the lines, "I don't know how Nixon could have won, because no one I know voted for him."
It's an older joke than that, my grandfather used to say it about FDR.
And, there is a lot of echo-chambering on-line.
An important thing to remember, when considering that one may be generalizing from their own clique or, similarly, appealing to popularity, is that, wild fad popularity that it enjoyed in the 80s, and resurgence that it's enjoying now, notwithstanding, it's always been kinda an insular, unpopular, hobby.
we all suffer from the belief that our experience is more universal than it is, and fail to see the ways in which it is not,
I get the impression a lot of folks quite enjoy it. They sure don't seem to be suffering, anyway.
In the instant case, you have a very strong attachment to a system that you believe allowed it to be liberating; that same system, for <me>, was constraining.
I think the clear takeaway, here, is that the "constraining" and "liberating" observations are not, in themselves, telling us much.
Ultimately, all RPGs are potentially "infinite games," capable of changing and expanding in play, with the possibility of successful play continuing the game, rather then being judged only at the end, by who won.
One might find details of a specific RPG "constraining," especially in a subjective sense, while others find different aspects "liberating."
Parmandur keeps reiterating, 5e has allowed for a massive explosion of people playing, especially new people and young people.
And returning ones, yes. It has /allowed/ the come-back to roll. Or been allowed to, by it's established fan base, who could have gotten in the way of it.
But, it's hard to say that this or that detail of the system is responsible.