why doesn’t the_content() work in this {single-custom_post_type.php} page?

Some post-related data is not available to get_posts by default, such as post content through the_content(), or the numeric ID. This is resolved by calling an internal function setup_postdata(), with the $post array as its argument:

<?php
$args = array( 'posts_per_page' => 3 );
$lastposts = get_posts( $args );
foreach ( $lastposts as $post ) :
  setup_postdata( $post ); ?>
    <h2><a href="https://wordpress.stackexchange.com/questions/28905/<?php the_permalink(); ?>"><?php the_title(); ?></a></h2>
    <?php the_content(); ?>
<?php endforeach; 
wp_reset_postdata();
?>

See Access all post data