I think the main reason would be the obviously mis-perception of the value of your company. I do believe people still look at crowdfunding sites like kickstarter and indiegogo as hobby or fun-type of investing as opposed to serious investment. Another reason would be the amount of investment dollars you are looking for and the unknown aspect of whether you could hit that target or not. With some crowdfunding platforms it's all or none in terms of hitting the investment goal amount and being able to actually use the investment, which poses a concern that if you ask for $1mm and you get $990k the entire effort could be lost.
All that being said, crowdfunding is an extremely viable resource and should be considered by everyone - at least for their early stages.
If you don't have enough backers yet, then don't crowdfund yet. You can't afford a very public failure.
If you are doing donation or rewards crowdfunding, make sure you have a tangible consumer product. It's difficult otherwise to crowdfund services, non-tangible or free products.
If you also want to patent, if you have secrets, it's better to make sure your business is secure before going public on a crowdfunding platform. Have a talk to your lawyer about this!
IMHO the main reason would be public failure. Having an idea targeted to a very niche audience is not a case for crowdfunding. Especially if you are not addressing to hipsters and geeks (aka early adopters). If this is making just your mom's life easier let's say..