Various reasons, first and most important: I don't want to use another application only for steam! KeePass and Tray TOTP (with some modifications) are more than capable enough to handle this task.
Second: for education, working with another developer's code, specially in a language you're not familiar with, its a difficult task but not uncommon situation.
Third: I wanted only to make a slightly modification, then I needed to make a new setting options, change the way plugin generate in order to add another step, some rewriting later, too much was changed for simply patch, so I decided to create my own fork and made more changes.