What are the reasons people don't believe in Digital marketing?

The very simple reason they don’t know the power of digital marketing for their business.
If a business person told about the benefit of digital marketing with ROI, it is quite possible he will sign for one such campaign.
We got success when we took a digital marketing campaign as free (honorary work) for some time to a Chemical Trading company. This company was getting 12 inquiries each month which is 10 fold now to 1200 per month by doing SEO of their web site, no doubt the web site designer took all our suggestion positively and we ask from our client to pay him Rupees 400/hour to this website designer though all content was provided by us.
Now this is our reference to other client pitching, seeing is believing, so we are getting good clientele and business people do understand the power of digital marketing for their business.