Because they are available for a limited time. No downloads per day means removal and this website's attach option failed miserably.
The bug you claimed to have been solved with the checkbox turned off/on for optimize buffer are not working with Sun-SSH2.1.0 or any non-recent SSH unix/linux implementations.
I have scrapped WinSCP completely unfortunately from our .NET solution and using pscp.exe from PuTTY works flawlessly with any SSH 2.0 library. There are a lot of threads regarding Fatally failed errors messages for big files that occur both in the GUI and .NET assembly with the latest build(s) - tried many, including 4.x versions - from unix to windows transfers.
I wanted to offer you some help in identifying the cause. All I can say now is that the bug still exists and for machines that can't be upgraded using WinSCP will fail.