Why can't Yandex or Google map be initialized from an included js file like common.js?

Question

JavaScript

Good evening. An interesting question arose. If you include the script of any of the specified maps in the head of the document, then it works, but if you take it out to a third-party file like common.js, it does not connect. Might be a stupid question, but why is that? Thank you.
I am posting this script:

ymaps.ready(init);
      var myMap, 
        myPlacemark;

      function init(){ 
        myMap = new ymaps.Map("map", {
          center: [53.677011,27.097172],
          zoom: 14,
          controls: ["zoomControl","geolocationControl"]
        }); 					
        var myPlacemark = new ymaps.Placemark([53.677011,27.097172], {}, {
          iconLayout: 'default#image',
          iconImageHref: 'images/i/map-mark.png',
          iconImageSize: [129,34],
          iconImageOffset:[-17, -64]
        });				
        myMap.geoObjects.add(myPlacemark);
      }