Bangladesh lacks players with T20 temperament, so Bangladesh has not yet learned how to bat in T20. Most Bangladeshi players bat in T20 like slow ODI cricket, while other countries bat aggressively in T20.
Sure thing. In T20 runs are more important than wickets and you have to take some risky shots to increase your runrate.
The ups and downs in smaller teams are bound to happen when a generation of players retire. Sri Lanka went through that low phase and had got back to some extent. They are still not as strong as their legacy but are still better than many teams. Bangladesh and Pakistan are going through that phase. The stronger the local cricket infrastructure is, the quicker the recovery is. Teams like Bangladesh and Pakistan should regularly hold local leagues and hunt newer talents as you can expect a different things from a team that has failed multiple of times.