Why did black avoid the straightforward capture here?
The extension is meant to attack the upper white group.

Looking at the whole board, the only solid territory black has is the upper right corner (the left side is open at the top and has an overlong extension at the bottom for territory-making purposes. The position is strong, which means it's good for fighting but not so good at making territory). The white, on the other hand, has a small but solid territory and influences on two sides. If black captures, white gets to attach at M18 and settle the weak group, in which case the strong black formation on the left side would be devalued by the lack of anything to attack. In that case, my intuition tells me, black would be behind (over-concentrated top-left, top left corner wall against a living group, the extension on the left closer to the stronger group versus the perfectly reasonable spread for white). The white group on the right would be slightly vulnerable, but not to peeping at 1 (see diagram below. Heavy ko with white capturing first).

$$Bcm1 Counter-attack
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O O O X . . . . . . . . . . . . . |
$$ | . . O X X X O . . O . X . . X . . . . |
$$ | . O X , . X . . . , . . . X . X X . . |
$$ | . O X . . . . . . O . X . O X 4 O . . |
$$ | . . X . . . . . . . . . . . O 3 O . . |
$$ | . . . . . . . . . . . . . . . 1 . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . X . . . . . . . . . . . . 2 . . . |
$$ | . . . , . . . . . , . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . O . . . . . , . . . . . , O . . |
$$ | . . . . . O . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------


However, after black extension at 33, a white attachment at M18 can be ignored by black indefinitely. J16 in particular looks like a counter attack. If white J17, black H16; white is dead (I might be wrong on this): 

$$Wcm1 Counter-attack
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O O O X . . . . . 1 . . . . . . . |
$$ | . . O X X X O . 3 O . X . X X . . . . |
$$ | . O X , . X . 4 2 , . . . . O X X . . |
$$ | . O X . . . . . . . . X . O X . O . . |
$$ | . . X . . . . . . . . . . . O . O . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . O . . . . . , . . . . . , O . . |
$$ | . . . . . O . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------


Had black captured instead of extended, white gets the hane at N17, which significantly increases white's eyespace:

$$Wcm1 With capture, white has eyespace
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O O O X . . . . . 1 . . . . . . . |
$$ | . . O X X X O . 3 O . X 5 . X . . . . |
$$ | . O X , . X . 4 2 , . . . X . X X . . |
$$ | . O X . . . . . . . . X . O X . O . . |
$$ | . . X . . . . . . . . . . . O . O . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . X . . . . . . . . . . . . . . . . |


In summary, aji is extremely important when you are attacking.