So now we as consumers use a lot of apps on our desktops, on web and smartphones (especially!)
This trend is sweeping software companies and IT departments themselves - we want only having deal with best tools solving specific problem and helping out to be productive + enjoy their look and feel. And it does not really matter whether this or that tool is open source or commercial - the most important is User experience, perception and passion to use that tool every day.
A quick example HP ALM (or any other ALM at this point on market) - a tool which can "do everything"... This tool attempts to fit all possible software entities (bugs, requirements, test cases, runs, test automation) into one sort of table view dogma. In essence this is boring, not domain specific - it is just like reflecting database structure on Web, bunch of fields and folder which can at least confuse and even scary. Sorry for negative example given first but it is really
The time for positive. Atlassian. This company create tools, specific ones, flexible, evolving. People love these tools. You can buy bundle, you can buy one tool. These tools are based on open standards, APIs, etc, so you free to build your ecosystem with tools and technologies of your choice. Let's go back to HP ALM, regarding integration, do they provide integration with CI, which test automation I can use with them apart from QTP, can I integrate with another nice bug tracker, do they support other requirements management systems? NO. Mercury tools gonna follow same mistake as Rational did after acquiring by IBM.
The problem of this kind a system/solution is not implementation, the problem is not even its cost, the problem is just idea itself, the idea of one-stop solution.
There are a lot and a lot great tools on market - a universe of them are commercial and bunch of them open source (especially technologies and frameworks)
Lastly, can you call Wallmart or Amazon one-stop store for you? Software market fragmentation is ongoing