Почему не переключает?

var but = document.querySelectorAll('.product_item .variant_select'); 
 
for (var b in but) 
    but[b].onclick = function(){ 
    var father = this.parentNode; 
    var a = father.querySelector(`[value="${this.value}"]`).getAttribute('for'); 
     
    father.querySelector('.weight_product').innerHTML = `Вес брутто*: ${a} kg`; 
};

<div class="product_item"> 
<select name="variant" class="fn_variant variant_select  " id="myselect"> 
  <option for="1.24" value="295" data-price="36,83" data-stock="50" selected="selected">1kg</option> 
  <option for="0.45" value="296" data-price="15,89" data-stock="50">300g</option> 
</select> 
   
  <div class="weight_product">Вес брутто*: 5.24 kg</div> 
</div> 
 
<hr> 
 
<div class="product_item"> 
<select name="variant" class="fn_variant variant_select  " id="myselect"> 
  <option for="1.24" value="267" data-price="38,26" data-stock="50" selected="selected">1kg</option> 
  <option for="0.4" value="268" data-price="18,30" data-stock="50" data-cprice="21,00">250g</option> 
</select> 
   
  <div class="weight_product">Вес брутто*: 1.24 kg</div> 
</div>