Query_posts works when appending via AJAX call wp_query doesn't?

I'm appending some posts on a click event to a twentyeleven child theme. For some reason though it only works when I use the query_posts loop and not a wp_query loop. For instance here's my wp_query loop:

$args = array('year' => $year,'monthnum' => $month);
$loop = new WP_Query($args);
if($loop->have_posts()) { 
while($loop->have_posts()) { 
the_post();
echo get_template_part( 'content');
} 
} wp_reset_query(); //doesn't work

Then here's the query_posts:

query_posts(array(
       'year' => $year,
       'monthnum' => $month
));

// our loop
if (have_posts()) {
while (have_posts()){
the_post();
get_template_part( 'content');
       }
}
wp_reset_query();

I've been told to avoid query posts like the plague so I'd rather use wp_query. Any thoughts on why this is happening?