Why script isn't downloaded when loading html via Ajax call

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Ajax

MVC

I write in MVC.
I have UserControl that has inside only this

UPDATED

<div id="login-container">
    <div id="login">
        <script src="/Scripts/shared/Site.js" type="text/javascript"></script>
        <input type="text" />
    </div>
</div>

When I load this control through Ajax call(I return View("UserControl")), so the script isn't downloaded. The only way to fix it is to write

$.getScript("Scripts/shared/Site.js")

after

$("#container").html(UserControlHtml)

I realize that this happens only when I load UserControl through Ajax call, but I don't understand why. Can someone explain what is going on behind the scene? I want to understand why loading html with script inside it doesn't downloads the script

Thank you