Can't get wp_dropdown_categories to work inside plugin

I managed to create an external page that shows a drop-down of all my categories inside my custom taxonomy called Frequencies that the user can select. The code is as simple as this (scrape.php):

<?php
define( 'WP_USE_THEMES', false );
require( '../wp-load.php' );
// delete the above two lines when using as plugin
?>

<?php wp_dropdown_categories(array(
                            'child_of' => 0,
                            'class' => 'postform',
                            'depth' => 0,
                            'echo' => 1,
                            'exclude' => '',
                            'hide_empty' => false,
                            'hide_if_empty' => false,
                            'hierarchical' => true,
                            'id' => '',
                            'name' => 'cat-dropdown',
                            'order' => 'ASC',
                            'orderby' => 'name',
                            'selected' => 0,
                            'show_count' => 0,
                            'show_option_all' => '',
                            'show_option_none' => __('None'),
                            'tab_index' => 0,
                            'taxonomy' => 'frequency',
                        )); ?>

The above displays the following options:

None
Daily
Weekly
Monthly

Now, I want to CONVERT the above into doing the same thing, but within the context of a plugin. The idea is the plugin asks the user to select a category and press submit to go to an options page.

So far I have created an admin.php page that contains the following:

<?php include(plugin_dir_path(__FILE__).'scrape.php'); ?>

So I have the plugin that displays a dropdown, but the dropdown only contains ONE option rather than displaying all the categories available under the taxonomy.

The output it as follows:

None
Daily

So the question is, what happens to the other 2 options?