Since WordPress 4.3, comments are turned off on pages by default.

This little plugin restores previous behaviour.

1.1

• Respect default article settings on Discussion Settings screen:
  • “Allow people to post comments on new articles”
  • “Allow link notifications from other blogs (pingbacks and trackbacks) on new articles”

1.0

• Initial release

1. Upload allow-comments-on-pages-by-default folder to the /wp-content/plugins/ directory.
2. Activate the plugin through the 'Plugins' menu in WordPress.