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]);