A lot of business application development requires a collaboration between developers and experts in the relevant area. One reason there aren't better small business accounting packages for open source OSes is that accountants don't tend to be interested in developing open source software. Of course, this only applies to the case of people who develop software for free, to scratch a personal itch, and for other reasons that do not involve being told to do so by an employer.
The other case -- people who develop software on orders from an employer -- is usually limited to people who are trying to make money off a rapidly-antiquated "software vendor" model, where the entire revenue stream depends on enforcing an (increasingly unenforceable) artificial scarcity condition so that people will buy something in units that, in reality, does not even meaningfully exist in units. For that case, you need a software vendor to decide to pay someone to make a product that can somehow be marketed to users of open source OSes, and the pathologically risk-averse nature of corporate bureaucracies tends to think of Linux as a gigantic risk not pursuing, regardless of whether it would cost anything notable to serve that market.
There is a third case, and that is in-house development. If a given business has the resources to develop a new software package for its own use, and sees some benefit in doing so, new applications can arise that way. The case tends to be that this is done by large corporations, however, and if they're going to do that for accounting software it's almost certainly not going to be something suitable for small businesses or individual home users. Worse yet, the control-freakish ways of corporate bureaucracies tend to confine such organizations to behavior pretty hostile to releasing software under open licenses unless it's part of a very specific business plan -- and internal development like that does not generally come with specific business plans involving releasing such software to the world under *any* license.
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