Because universal salvation isn't the only question of religious truth in the world. (Or non-religious truth.) Duh.
Also, universal salvation for many of us (myself included) is connected strongly to various other theological and/or emotional ideas, which can be in mutually exclusive conflict with other
theological and/or emotional ideas connected strongly by people to universal salvation.
To pull one example out of a hat: are we supposed to be worshiping God? If so, how? And should we be worshiping any lesser lord or God? If so, how is that to be reconciled with very strong and frequent exhortations that we shouldn't religiously be doing that? If not, why do the scriptures expect and exhort us to religiously worship Jesus Christ and the Holy Spirit (in the NT) and the Angel of the Presence (in the OT)? Aren't they persons in distinct personal relationships with God the Father? -- and if they aren't, then why do the scriptures often (and very very regularly for Jesus) treat them that way?
Those are the same issues Biblical Christians have always
been chewing over, and chewing each other, about. Being saved from our sins involves, at least partly if not chiefly, being saved back into proper loyalty to God and out of putting something less than God more importantly in our lives than God; universal salvation thus involves that for everyone, sooner or later. (And how much sooner? -- how much later? --why later, if later, and why aren't things already so much better if sooner? Are we doing something wrong? We ought to be correcting our mistakes, but what are our mistakes?) But what does loyal worship of God involve and not involve? And where it involves serving other people, what does that involve and not involve?
Then comes the question of acting to inconvenience other people: is that ever right? If so, to what extent? God sure looks
in the scriptures like He's acting to inconvenience creatures, sometimes innocent ones along with guilty ones, to various degrees, even to death, and even after death? How real are those threats, or if real what do they really involve? Were
those threats real but are permanently over now?
-- and if so, why? Should we be worried about ourselves or the people we love, and if so why and to what extent, or why not? (Should we be worried about people we hate, and if not or if so then to what extent?!) A lot of universalists have strongly emotional reactions to things they felt threatened with which (they or we think) turned out not to be true to one or another degree, and rightly or wrongly their universalism is thus strongly connected to opposing the threat of those threats to one or another degree.
By extrapolation, opposing the threat of those threats can, rightly or wrongly, involve opposing ideas which other universalists still see, rightly or wrongly, to be variously levels of important. That can lead to a lot of passionate arguments among universalists in itself, not apart from the question of truth, but supercharging the intensities of the disagreements about what truths are real and/or important -- including important for universalism to be true!
And all that is charitably assuming that everyone involved on every side happens to be acting in a perfectly charitable and otherwise sinless way! -- but people are sinners (right? or wrong? or under what conditions? the answer either way would be an important truth to take account of!), and as (if?) sinners then we may be opposing each other on points of disagreement about truths, in ways which are unjust to other people. Which, whether those injustices are large or small (or only perceived wrongly as injustices, large or small), doesn't make the disagreements more naturally peaceful.