[jquery] How to set selected value on select using selectpicker plugin from bootstrap

Actually your value is set, but your selectpicker is not refreshed

As you can read from documentation
https://silviomoreto.github.io/bootstrap-select/methods/#selectpickerval

The right way to do this would be

$('.selectpicker').selectpicker('val', 1);

For multiple values you can add array of values

$('.selectpicker').selectpicker('val', [1 , 2]);