[I'm not a "small biz owner" but I founded isocket / BuyAds.com, which helps small biz owners advertise. Also previously founded a marketing consultancy for SMBs]

The common reasons SMBs don't / aren't investing in marketing:

* Education.
Most people know marketing from being on the receiving end of a message, but not the giving end. Which can even be dangerous, because then a SMB might apply the wrong principles ("I see TV ads, so maybe I should buy TV ads too!")

* Steep learning curve.
Given the lack of education and learning curve of proper marketing, SMBs who approach marketing get overwhelmed. It becomes obvious that doing it right takes time and money, which is something SMBs are short on.

* Lack of measurability.
The more attributable a piece of marketing is, the more likely it's to be seen as successful by an SMB. But most marketing, especially the advanced forms beyond billboards/etc, is hard to properly measure. That, coupled with the fact "50% of marketing dollars are wasted, we just don't know which half" (famous agency quote), results in SMBs feeling like its a black hole of money.

* Difficulty making creative assets.
Good advertising needs good creative (the thing a person sees/hears, like a billboard). Making good creative is hard and thus inaccessible to many SMBs. You don't want your nephew who kinda-knows Photoshop to build your banner ads for you.

* High cost barrier to entry.
A SMB might be able to spend $1,000 on online marketing, which might be a big expense for them, but is a drop in the bucket compared to other markets. Sometimes, smaller SMB budgets can be ineffective at getting enough "share of voice" to make an impact at all.