We have to make it compile on RHEL 5 and RHEL 6¹ because some obscure SCSI drivers only work on those platforms, and you need those if you're going to virtualize the hardware (P2V). You can't just use modern Gtk on those platforms partly because it doesn't compile and partly because carrying the whole "G stack²" on those distros for a 3 dialog wizard would be a nightmare.

¹ 2007 and 2010 respectively, not sure off the top of my head what versions of Gtk those have, but old.

² It's not just Gtk, but glib and dozens of other libraries.