Why are Flipkart/Snapdeal/Amazon giving discounts and deals only on mobile apps and not on their websites? What is behind this new marketing strategy?

As we know that mobile app is a very effective tool to connect with the customers because it provides easiness to the user & also it shows how many customers or users are using the apps.

To be in the competition these rivals are sending notifications on regular basis to the users & trying to motivate towards the product.