This is how I see it after working in a retail environment.
From a fortune 500 retailer point of view: They have IT staff dedicated to the POS systems and their inventory management system. They know how to make the POS systems fly. Oh and their supply chain software integrates with the inventory software, can you're product do that? No? Sorry we're going to pass. Not only that, they have tons of data warehouses of sales data going back to who knows when that the marketing people get to play with. They also may have a stringent budget and purchasing process and are most definitely led by non-technical executives. There is the old adage "You get what you pay for" and if you don't have to pay for the software then what do you get? They also have many locations (lets say 2000 for simplicity) all running the same registers. If your software doesn't support their registers, you can bet they are going to pass on you. Plus they have the advertising money to spend on AdWords campaign or paid search if need be.
Now a medium sized retailer a handful of locations. They are interested because it will allow their products to be found by those and they may be able to poach a few customers from the big box stores. Alas, but they too have existing POS software and inventory management software and would be willing to ditch them for something new. However, it may be hard for them to be the first to test out a new product since they rely on their customers and know that they could probably get the same items cheaper from a big box store. If there is any snag in their experience at their store (such as POS issues) than they could lose that customer for good. They are hesitant but would accept once it is proven and stable.
Then there are the small mom and pop stores. These may want to use the products but they don't have the infrastructure to support it. Heck, they are currently doing inventory in Excel and hate it, this would free up their time. But that free software isn't free when you need to buy new computers and registers to be able to support it.
Notice that I keep saying POS instead of just inventory management. The reason being is that those two need to integrate with one another and if you want live levels of stock at any given time, you will need to use the POS system as inventory management software is by the day (at least from what I've seen).
Plus, you get to the lock in factor. Is the software going to be free if I don't keep paying for the search stuff? What if I decide that it isn't working for us, do we have to switch software or can we keep using it for free? If you allow them to keep using it for free then there goes some profits. If you don't allow them to use it for free, then you get into a situation where it becomes a super expensive test since the software doesn't cost money but it costs time to set up the new software and train everybody on it and then if they don't like it switching back.