Dash has a treasury system which has 5-6 million in funding per month to grow the network and its value with new integrations, projects and marketing. LTC does not.
Dash has instant send which makes it perfect for Point of Sale transactions. LTC does not.
Dash has Privatesend to make anonymous transactions. LTC does not.

The main reason is the Dash scaling roadmap and Dash Evolution - this will bring Dash into a new era of mainstream adoption. If you have not looked into it yet, I suggest you do.

In 2018 Dash will surpass LTC very quickly as no coin currently has 5-6 million a month to grow itself. Thats a big ass snowball rolling down a hill that cant be stopped.