Why is my mobile conversion rate lower than desktop?

Because mobile carries the browsing and desktop carries the deciding. Part of the gap is real friction and part is that the two devices are used at different moments in the same purchase.

The reasoning

Separate them before fixing anything: look at mobile sessions that reached checkout against desktop ones. If the drop is at checkout it is friction and fixable; if it is earlier, mobile is doing its job as the browsing device and the fix is bringing people back.

What most people get wrong

The slowest part of most mobile stores is images nobody resized. It is the most common performance problem and the easiest to fix.

Discounting to grow trains customers to wait for discounts. It works once and costs afterwards.

Traffic is the expensive problem and conversion is the cheap one. Doubling a 1% conversion rate is usually easier than doubling visitors, and costs nothing per month.

How an app changes it

Push notifications are the one growth channel you do not rent. Every other route to a customer is paid for again each time.