This is very simple. Email marketing is extremely cheap as no paper is involved. The cost of sending an email marketing note is EXTREMELY low. Furthermore, if your list of email recipients is well-targeted, you can drive significant business with this marketing strategy. However, many people use email poorly.
You can’t just buy a HUGE list an expect that those people will turn into customers. This is not how it works. You need targeting. And after you have targeted, you need to nurture. People don’t buy things at first glance – they need to be brought down the decision path – with confirming evidence that they are making a good decision along the way. This takes time and the right approach. Email marketing can be a great part of this strategy, but it needs to be done well.