A few factors, from what I’ve seen.

Price - They think that cyber security is too expensive. 

Knowledge - They are sometimes led to believe that either their computer has adequate protection already on it, that their modem has a firewall built in, etc. 

They Don’t Think They Need It - “Who wants my data?” Is a response I get quite often.

We offer multiple levels of protection from Gateway Security Appliances to Remote Management and Monitoring for our customers.  Data loss can be a big problem with small businesses.  I have seen companies go out of business from a small data breach.  Nevermind a large one.