Firstly, the firm does not have to install the solution on to its systems. Installation is often complicated and integrating a new tool into the existing work flow is even more so. There have been instances where the entire existing system had to be overhauled just to make way for the new solution. That's impractical and very undesirable.
Next, on demand ecommerce solutions are maintained and upgraded by the vendors themselves. So the subscriber firm need not worry about periodic software upgrades from its own end. Also, because a vendor rents out its hosted ecommerce solutions to more than one subscriber at a time, the feedback it receives lets it identify which aspects of its service are being well appreciated and which are the ones that need a re-think.