uninstall.php does not appear to trigger when uninstalling my plugin

i am new to plugin development in wordpress and i have have this simple test plugin that i am working on. The problem i am having is that the uninstall.php file does not appear to get trigger and as such the database entried i wish to remove remain after the plugin in uninstalled from the WordPress admin.

Plugin code:

defined( 'ABSPATH' ) or die( 'Looks like you made a wrong turn there buddy' );

class TestPlugin
{

    function __construct() {
        add_action( 'init', array( $this, 'create_post_type' ) );
    }

    function activate() {
        // generate a CPT in case 'init' fails
        $this->create_post_type();
        // flush rewrite rules
        flush_rewrite_rules();
    }

    function deactivate() {
        // flush rewrite rules
        flush_rewrite_rules();
    }

    function uninstall() {
        // delete cpt
        // delete all the plugin data from the DB

    }

    function create_post_type() {
        register_post_type( 'acme_product', array( 'public' => true, 'label' => 'Acme Product' ) );
    }
}

if ( class_exists( 'TestPlugin' ) ) {
    $newTest = new TestPlugin();
}

// Activation
register_activation_hook( __FILE__, array( $newTest, 'activate' ) );

// Deactivation
register_activation_hook( __FILE__, array( $newTest, 'deactivate' ) );

Uninstall.php

/**
 * Trigger this file on plugin uninstall
 *
 * @package TestPlugin
 */

if ( !defined( 'WP_UNINSTALL_PLUGIN' ) ) {
    die;
}

// Clear database stored data
//$acme_products = get_posts( array( 'post_type' => 'acme_product', 'numberposts' => -1 ) );
//
//foreach ( $acme_products as $acme_product) {
//    wp_delete_post( $acme_product->ID, true );
//}

// Access the database via SQL
global $wpdb;
$wpdb->query( "DELETE FROM wp_posts WHERE post_type = 'acme_product'" );
$wpdb->query( "DELETE FROM wp_postmeta WHERE post_id NOT IN (SELECT id FROM wp_posts)" );
$wpdb->query( "DELETE FROM wp_term_relationships WHERE object_id NOT IN (SELECT id FROM wp_posts)" );


echo "UNINSTALL DAMMIT!!";

UPDATE:
I have tested the SQL query just to make sure it worked in phpMyAdmin, and it works fine.

i.e "DELETE FROM wp_posts WHERE post_type = 'acme_product'"