Because they need to change SIM's all the time. There are websites listing suspicious numbers, simply Googling the number reveals if the number is is used for scamming. Unsuccessful or successful attempts often result someone writing about it in one of those phone number websites.

I don't know if the SIM card limit is by national ID number or operator but there are not that many GSM operators in Bulgaria anyway, it's a small country. There were more before but they merged and now there are 3 in total.