"Write once, run everywhere" is pretty important goal to achieve. When a project has to be written for a platform, and then ported to other platforms, then some ports or all of them turn out to be pretty bad.
Essentially the program runs only on one platform well. The ports also may turn out to be a significant burden depending on the size of the project, which usually grows over time.
Take Adobe as an example. It's port of Linux was pretty much unusable, only many years later to be revealed that only 2 programmers were responsible for maintaining the codebase.
