Because they want your money, it's as simple as that.
Now that you have allowed Sparktrust to scan your pc, I would recommend you remove it via Add/Remove programs and whilst your looking at the list see if anything else has installed itself unbeknown to you.
I would suggest you also run a scan with Malwarebytes free, too make sure all is well
https://www.malwarebytes.org/mwb-download/
When you install, opt out of the free trial version and any other additional bits.