Q:

Why was it better to abandon the boat?

I did not quite understand the reasoning re abandoning the boat in The Revenant. To my understanding, travelling in the boat would have removed them from the pursuing people faster than those pursuing could consistently ride (a galloping horse would probably have been faster than a drifting boat, but horses cannot maintain that speed for long).
How could the pursuers even maintain the speed needed to catch the boat?
(Note, one possibility is if the river took a long loop around a thin neck of land, that horses could cross in much lesser distance/time than the boat that had to traverse in its looping course. But I don't think anything like that was mentioned in the movie.)

A:

Can horses outrun a boat?
That plot point does seem to hinge on this question. Apparently the boat served as at least a temporary escape and could not be immediately followed by horseback pursuers, perhaps due to the river passing through a swamp or gorge.
But boat-escape only buys a little time if horses are ultimately faster over distance. Then the Arikara could choose a location downriver suitable for ambush and wait for the boat to pass. This eventually happened in the movie; there was a brief scene of arrow-skewered white people on a boat. So back to the question.
Of course all kinds of variables are at play, terrain, river speed, horse diet. And the whole thing is semi-fiction anyway. But here are some grossly feasible numbers.
A horse can cover 10-100 miles a day off-road. 10 miles walking leisurely through un-blazed mountain passes, 100 miles by a Mongol soldier with 3 or 4 horses.
A navigable river flows about 3 miles an hour, about walking speed. That's 72 miles a day if you float around the clock (and pursued like that who wouldn't). But as you said, the river might not be straight. In fact, most rivers meander. The Connecticut river near where I live is relatively unkinked at 410 miles from source to mouth, but it's only 280 miles as the crow flies. So 72 river miles is about 50 crow miles (50 ~ 72*280/410). 
Assuming the 10-100 horse miles are relatively straight, they are comparable to 50 boat miles, and there is no obvious winner. 
But my bet is on the horses. Though the trappers were motivated, there's no hurrying the boat they were on. Horses can be motivated, especially when ridden by a father with some authority trying to rescue slash avenge his daughter.

