Trips on flop and capping when you know they have a frush

[Schizo]

Retarded noob question:

I know when you hit trips you are supposed to cap a monotone flop against 3 players. With 7 outs you are getting 3:1 on 2.6:1 odds which is +EV. However, I've seen very good players cap on a monotone flop vs 2 players. Why?

for example:

Preflop: Hero is CO with 4[img]/images/graemlins/heart.gif[/img], 4[img]/images/graemlins/club.gif[/img].
UTG folds, MP folds, Hero raises, Button calls, SB calls, BB calls.

Flop: (8 SB) 4 [img]/images/graemlins/spade.gif[/img], 7 [img]/images/graemlins/spade.gif[/img], 6 [img]/images/graemlins/spade.gif[/img] (4 players)
SB bets, BB folds, Hero raises, Button calls, SB 3-bets, Hero caps, Button calls, SB calls.