The *only* reason? No.
I can say it is narrativist! In the sense of supporting a better narrative - the Tarrasque moves as fast as is necessary to make a good story. The story is better, more tense (and more apt to continue) if that last adventurer gets away...
I've known many players who have a term for this - "moving at the speed of plot".
http://tvtropes.org/pmwiki/pmwiki.php/Main/TravelingAtTheSpeedOfPlot
So, no, it isn't really only gamist. Don't be so quick to label things.
Yet the reason why the adventurer gets away (unless other creatures join the combat, even if they are on his side) is not because the DM decides that the narrative would be better (although he might certainly believe that, too), but because the rules say that under this circumstances the Tarrasque moves slower.