John is a fascinating philosophical take. Its perception in scholarship has also evolved remarkably. Pretty much everything we thought we knew about John has changed in the 20th century, particularly since the 1980s. There has been new knowledge about John that has come from excavation of the pool of Bethesda to the relationship between the "children of light" and the "beloved teacher" in the documents uncovered in the Qumran scrolls from that community. I'll frame my answer as someone who has gone through a religious trajectory that has included traditional mainline protestantism as well as universalist frameworks.
The distinction between universalism and particularism seems to bend around two separate takes. Calvinists, for example, believe in particularism, but that we have no say in the matter at all. This is called double predestination. From the beginning of time, everyone is already sorted into Heaven or Hell, and it is in no way up to us. It is important to note that Calvin was not a determinist. He believed we had enough free will to warrant all of us going to hell, and that it impossible for us to work to develop merit to goto heaven. Salvation was entirely up to God and was already determined. Today, through the theology of Karl Barth and others, the Presbyterians (those who inherit the Reformed church dogma of Calvin) have more of a universalist bent (at least in the PCUSA branch).
This particularism is contrast with the catholic, methodist, episcopal, or evangelical doctrine that it is up to us to choose to accept Christ as our own savior. In this mind, we get to choose. And not everyone chooses. So only a particular set goto Heaven. This creates what we call the protestant work ethic upon which the USA is built. Unlike with Catholics, where a priest is all you need for salvation (and he will tell you this), for the protestants, we can never know if we are saved for sure, so we work work work...
Given those two kinds of particularism (and they have their defenders in the Bible), Universalism stands in contrast. For a universalist, Jesus descended into hell, kicked open the doors, and all are saved. For the universalist, salvation is also not in our hands (as with the Calvinists).
In John, support for this is drawn from the theme of John 1:12-13 where the text says that "for all who received him, he gave the power to become children of God, but not by their will or the will of others, but by God." Then in John 6:44 and John 14:6, we have this paradoxical pairing of statements. In 6:44, no one can come to Jesus unless they are "dragged against their will" by the father.
The verb here is "ἑλκύω" and is a technical term in John used exactly six times. I suggest exploring those uses as it is enlightening and talks much about our ability to participate in our salvation. The BDB Lexicon has: "drag a person forcibly and against his will"
And in John 14:6, we have the famous statement that no-one comes to the father except through Jesus.
So there is this paradoxical framework that says that none of it us up to us. In fact, you might say that the idea that we have our own free will or merit is questioned entirely in John. The only way is through Jesus, you can't come to Jesus unless dragged against your will by God, and then in John 12:32, we have Jesus' statement that "if he is lifted up [on the cross], then he will drag all to himself." Again, the verb "to drag against your will" is used here, as well as the greek word "πάντας" meaning ALL.
So with that trio of verses, you get this round rejection of our role in salvation and a statement that God is in charge of it, especially when coupled with the theme in John 1:12-13 that it is not up to us.
Some scholars see a "protology" in John. This is that John points back to Eden. Instead of being born of blood/flesh, we are to be born of water and spirit, just like Adam was. Instead of eating food from the cursed ground, we are to eat food from above (end of John 6). These were the major punishments upon exile from eden. And the "sin" in eden was to reach out and grasp at something against God's will. But that was also paradoxically, the knowledge of right and wrong itself. So before this act, we could not have known right and wrong and would not have been moral agents in the act.
Compare this idea to what Jesus says it means to be a child of God in John 5:19, "Jesus said to them, “Very truly, I tell you, the Son can do nothing on his own, but only what he sees the Father doing; for whatever the Father does, the Son does likewise."
There is a sense here that Jesus is utterly obedient to God. This again is in contrast to the seeming disobedience in Eden. Furthermore, it would seem that the only reason that a person could reach out to choose to be saved in the first place is if they thought this was a "GOOD" thing to do... which, in the eden context, is more fruit of the knowledge of good and bad. The concept of "dragging against the will" and making it clear that we cannot do it ourselves removes the idea that we are chasing it because we think it is good, or chasing it because we are avoiding hell which we think is bad.
In fact, there is simply no other reason that we can move as beings. We only act to achieve what we think is good or avoid what we think is bad. So it is literally impossible for us to act outside of the knowledge of good and bad. Hence you get a take that it simply cannot be up to us that is well supported in John.
I'll frame this with some meta-commentary by saying that universalism has persisted throughout Christianity (Origen being one of the first recorded explicitly open Universalists). But it is extremely hard to sell this. There is no value proposition behind Universalism. You walk into a universalist church and they say, "nah, you're good, all are saved." Then you walk out and don't come back. It takes the fear of hell to keep you in the pews, which is why the Universalism doesn't play well. In the early history of the USA, there was a huge backlash against Universalists and Calvinists in a very similar way to the backlash against Atheists. There is an idea that if it is out of your hands, then there will be no motivation for these people to be moral.
That is another long take, but it informs why it is challenging to find broad institutional support for Universalism even though it may be well supported in the text.