Comment by cjs_ac
For non-technical users, the user interface is the program. To them, there's nothing beneath the shell. My last boss didn't like Macs because his PowerPoint presentations rendered differently on them compared to Windows. There are millions of real people with consequential positions in important organisations who think like this.
Getting your program to render the same on different platforms takes a lot of work the user will never see, and that goes double if you actually want it to behave the same. There are some very deep problems you have to work through, like what behaving the same and rendering the same even mean. Same window decorations? Same menu layout? Same dialog boxes? How are you doing font rendering? How much do you embrace platform defaults versus steamrolling over them in the name of uniformity?