Cause SMS is not encrypted, so your phone provider and any routing company like this one can read your texts, phone numbers can be spoofed (not sure about this one), or stolen via social engineering (someone calls your phone provider claiming it's you and u lost your phone or something, then gets access to your number).
There's probably a lot more issues with it, i don't remember them atm, but should be pretty easy to find with a search if you're still curious.