Because it relies on exploitation. It offers pennies for work that ha real risks involved. No insurance, not even bike maintenance is offered. You're on your own and if you happen to be the unemployed, unprivileged youngster from the outskirts of Stockholm (quite common) and anything happens while you're riding for one of these companies (car hits you, pedestrian walks out in front of you, etc.), at best your bike is busted and you can't even pay back the money you borrowed to buy it just to get the job. At worst you'll have serious injuries and you'll need treatment that you'll get for relatively little cost only because this is Sweden not one of those fucked up countries where health care is terrible and you'd go bankrupt if such a thing happened to you.
That's why the mode is poor. It makes me sad that all these delivery companies build thir business on exploitation.