Most likely because it's a discussion on a subject that affects people. People will find it hard to discuss something objectively when they themselves have been personally hit by a government policy.
That and partly ignorance. Politics as a whole is great at giving everyone an opportunity to spout any close mindedness easily and pass off generalisations as if they're correct.
That and as Whimsical Sloth said, people hate being proven wrong, and it's quite difficult to break down an argument tinged with emotion and subjectivity than an objective, factual one. Hence why so many do it.
It's definitely something people have to learn, to be purely objective.