Because to them it isn't a matter of equal rights.
There are two languages being spoken here, and the gap is causing animosity. To one side, "marriage" is just a word, something we can define as we need -- but to the other, marriage is something defined by God and handed down, not just a ceremony, but an expression among humans of God's own Self, an illustration of His nature and being, a step toward sharing God's presence in a small way.
So when it's proposed that "marriage" be changed to mean something other than that, the reaction isn't on the level of reason; it's a visceral response on the same level as if someone drove a manure truck to a dedication ceremony for a facility in memory of Matthew Shepard, with an all-gay band on the stage, and sprayed the entire place, people and all with fermented cow shit. It's akin to armed robbery where the perpetrator pisses on the children before leaving.
Yet those don't do it justice, because it's more to them as if those things just kept going, as if for us someone dumped manure in every gay club, bar, or other business, and every gay activity -- all the time, every day, without let-up; as though every time you turned to kiss your bf, a bucket of piss were dumped on you from above.
So one side says, "We want equal rights", but what the other hears is "We're planning to shit on your most precious stuff".
That's why I believe we need to get the word "marriage" out of the picture -- and not only that, but to acknowledge that marriage is a religious matter, and that the government will not seek to change it.
THEN we can address the issue of freedom of association, acknowledging marriage as one kind of association, but insisting (on the basis of the supreme authority of the land ordained by God) that all similarly deep sorts of association must have equal standing before the law.
