4 records out of 6 records in the database return successfully. But two records are returning unsuccessful. When I look through the controller, there is no problem with the data. It also loads very slowly even though there are only 6 records. How do I solve this problem? Is there a more alternative way for me to pull the data?

Client Side(Javascript) Slowness:

Now let's look at your javascript/jQuery code snippet"

The controller already return the data as json. But what are you doing here:

var item = jQuery.parseJSON(data);

Then again: $.each(JSON.parse

Do you know the how costly it is? Have you heard of Big O? It will cost O(n^2).

Efficient Way:

<div>
    <table id="table-maincategories">
    </table>
</div>

@section scripts {
    <script src="https://ajax.aspnetcdn.com/ajax/jQuery/jquery-3.2.1.min.js"></script>
    <script src="https://cdn.datatables.net/1.11.3/js/jquery.dataTables.min.js"></script>
    <script>
        $(document).ready(function () {

            $.ajax({
                url: "/Admin/BusinessCategories/MainCategories/",
                contentType: "application/json",
                dataType: "json",
                type: "Get",
                success: function (data) {

                    $("#table-maincategories").empty();
                    //Its already in json so no need to parse again
                    $.each(data, function (index, element) {
                        console.log(element.id);
                        $("#table-maincategories").append(`<tr >
                                                        <td><img src="${element.Image}" style="height:80px;" /></td>
                                                        <td>${element.Name}</td>
                                                        <td>
                                                            <div >
                                                                <a href="/Admin/BusinessCategories/MainShowMenu/${element.Id}" onclick="location.href=this.href;">
                                                                    <input checked="${element.ShowMenu}"  type="checkbox" id="flex_${element.Id}">
                                                                </a>
                                                                <label  for="flex_${element.Id}">Menüde Göster</label>
                                                            </div>
                                                        </td>
                                                        <td>
                                                            <a asp-action="EditMainCategory" asp-route-id="${element.Id}" ><i ></i> Düzenle</a>
                                                            <a asp-action="DeleteMainCategory" asp-route-id="${element.Id}" data-name="${element.Name}" ><i ></i> Sil</a>
                                                        </td>
                                                    </tr>`);
                    });
                }
            })

        });
    </script>
}

Note: You can see only one each loop $.each(data, function (index, element) {} is enough what you are trying to achieve

Reason For Undefined:

Here, debugging will be your savior. From your details its hard to know the reason. You can debug as below:

Note: As your reason is unknown so debug will provide you the clue why only two data is not loading.