There are two main reasons.
First, the payments have fees associated.
If there was no minimum, there'd be some cases where the value you'd get would be very low.
The minimum of 50€ makes it all worthwhile.
Second reason?
This is an internal decision which allows us to better manage our time.
Can you possibly imagine 3000 affiliates asking for a 20€ payment every single day?
We're Mobidea Crew members, not robots!