5 simple straight forward reasons really.
1. West Indies bowling was piss poor - Apart from Rampaul , every other bowler bowled 'club level' length bowls and our Batsman took good advantage . West indies relied too much on Sunil 'T20' Narine who has been a major flop for them this series . Roach was poor and Andre Russel was and is a mediocre bowler who gets mostly 'gifted by batsman' kind of wickets.
2. Failure of Gayle and Samuels . Gazi's bunny Gayle didnt have a good time out there . He wanted to 'play off' gazi's over and attack the other bowlers. Mortaza didnt give him that and Gayle had to go back to the pavillion Gangnam style. Samuels at this very moment must be mentally preparing himself to counter Gazi but I dont see him being successful in the next matches.
3. Anamul Hoque
4. Khulna Pitch and the inability of west indies batsman to play good quality spin.
5. Shohag Gazi and Abdur Razzak - 13 wickets (65%) between these two . Tells you the whole picture really.
Hope Dhaka pitch plays the same as Khulna.