First off, I like the amount of passion you have for the subject!
Now for the answer:
VoIP is a digital way of sending voice communications using IP.
An example for this would be sending voice over the internet between two VoIP nodes.
Because VoIP uses the internet to communicate, the access to the nodes and to the servers handling the requests are at risk to the whole world. This means that if I scanned the internet and found a VoIP node and I found a vulnerability then I can use the node's OS for any malicious purposes (Example: using it as a bot in a DDoS attack, sniffing the traffic and there for listening to the calls and etc...).
On a traditional phone line the communication is done via a private closed off network that is wired to your house from a control box that is probably somewhere in your neighborhood (And that box is connected to another and so on until the main control system).
In order to hack that you would have to connect to this wired network. This requires physical access which already makes it harder to hack then a VoIP network.
There are many more small reasons and each have there own good and bad sides but in general the reason VoIP is more vulnerable is because it uses the internet or any other network to communicate which makes it vulnerable to all standard attacks. Either way, they are both pretty hackable each in their own way.
Note: Some service providers have already mixed together there control systems for the phone lines with the internet making it also vulnerable.