The reason I believe they aren't successful in the business world is that they killed the clones that would have enabled higher volumes with cheaper options.
...which hit Apple's own sales of premium-priced laptops, workstations and small-form-factor systems, the income of which pays for the development of OS X. And the cloners didn't go after the 'standard desktop PC' market because, as you say, Windows had a stranglehold on that, they went after Apple's premium products which are more profitable and had proven market niches.
Anyway, there's hasn't been any money in "standard PCs" for a long time - they're loss-leaders for selling other goods and services.
The other reason to be honest is that they don't have Windows. Businesses stick with what they know
In more practical terms, businesses rely on lots of custom software (mainly customised databases) often, shall we say, not written to the highest standards of software architecture? The lack of SQL Server, Access, Visual Basic, Visual C/C++/C#* etc. is a deal breaker, and Mac OS <=9 didn't offer much by way of equivalent (Filemaker was great for small biz but didn't compete with the big stuff). OS X can run serious database software - but only the same stuff that runs more cheaply on Linux.
(* Yeah, I know XCode rocks for writing full-blown Apps, but MS Visual XYZ hits the sweet spot for commercial database programming).
Where Mac rocks is, for example, small-scale website/web app development for deployment on linux/unix servers, where you can have Adobe CS (or similar) and MS Office alongside apache/php/perl/python/ruby/mysql/postgresql running in a Unix environment as nature intended... but that's not how big business rolls.
Why is iOS becoming successful in business?
Because the Pointy-Haired-Boss buys an iPad 4 from his local Apple Store then demands that the IT department make it work with the company network.
Then they reason that if they support iOS, all their employees will rush out and buy their own iPhones and the company won't have to buy them all CrackBerries.