It varies.

A lot of the time when you see it happen, the other side isn't exactly being polite either. Its a battle of "snowflake" vs "moron". In those cases, theres much more at stake admitting your wrong, with much more permenance. You're essentially admitting that not only were your theories on climate change wrong, but that the oppositions view on you is correct, you are a moron. Neither side means this, of course, but thats how it ends up being.

In more polite arguments, denial could down to fear of change, fear of being wrong (essentially the same as the less polite argument, but with the loser thinking that of themselves), or perhaps even just simply not being convinced for whatever reason