If you sell your software as SaaS, you need to make sure it is something that your customers use every day. This is a problem when you sell to consumers, because often they will need your software only a few times a year.

I sell a product named MDB Viewer on the Mac App Store, and judging from support emails many of my customers need it just to convert a single database; but they are happy to pay, and they like that they own a license. There's no way I could sell this as a SaaS product.