Because theymos want to code with things he is most familiar and comfortable with to save his time.
It's been a decade since they modified this forum software. Too many changes were made in this forum software which maybe we did not notice. But, Upgrading to a new software would require modifying those features again, which is too time-consuming and theymos doesn't have it. Of course, he will be familiar with new codes as well once he starts to play with the new software. But, once again, He doesn't have this time.
- I am the only bitcointalk.org sysadmin and on-demand programmer, and I'm used to the current software. Furthermore, I need to frequently make changes to the current software, but each change I make might require alterations to Epochtalk, which is problematic.
Now a question is spinning in my head. As he said, he id the only sysadmin and on-demand programmer. What will be happen if we unfortunately lost theymos? This may doesn't make much sense but the possibility is unlimited.