Have you all found that everyone maintaining their own linux os and configuration is easier to support than a standard virtual appliance that can be deployed and provides a unified OS and backend configuration? From an ease of use perspective now days it's much more preferred to deploy a pre-built appliance that will also provide a reliable upgrade path that customers don't need to deal with.
Look at some of the examples below that made their prroducts much easier to manage by providing pre-built appliances. Some of these vendors may have software that only runs on linux but they sell to a windows only shop. That is a great advantage and plus for those customers so that they don't need to invest in expertise for just a single product.
Vmware (so many appliances)
Cisco
Tenable
Various AV Vendors/Content Filtering Vendors
Load Balancers
I suspect if the SaaS offering is much more popular than on-premise tthen that may be why investing in a standard appliance has not received as much attention.