The main reason for this is that targeted promotion or product specific landing pages are focused on a single objective that matches the intent of the ad that your visitors clicked on to reach your page.
If you consider the example of sending traffic to your homepage vs. a standalone landing page, you can understand that your homepage is designed with a more general purpose in mind. It speaks more to your overall brand and corporate values and is typically loaded with links and navigation to other areas of your site.
Every link on your page that doesn't represent your conversion goal is a distraction that will dilute your message and reduce your conversion rate.