The older the code is the more difficult it is to make updates to the project especially when you have different coders come and go through the history of the site. The people who originally made BOM haven't been there for years. It's part of the reason IMDB shut down the Box Office Mojo forums/derby (the old code led to a crash of their log-in system which IMDB never fixed due to the age). It's less of a problem if you have a big staff who can make sweeping changes to the site or the same coder but Box Office Mojo had neither. Keep in mind Box Office Mojo isn't pure HTML: it's partly an online database which meant lots of complex code.
I could be wrong but I'm pretty sure Box Office Guru is just HTML (which is super basic code) and maybe some basic CSS (which is just a visual template for the website). It's functional, sure, but Gitesh isn't doing anything complex with his website. Even though he has a "database" it's really just him editing the webpage with the data rather than an actual web database. But like I could easily jump in and take over and not miss a beat with his coding-and I only know very basic HTML/CSS! I couldn't work on BOM's code (or even BOT's code which is also immensely complicated!)
It is very good for BOM to get an update to it's code (and look) but it's bad that they changed the nature of the website in the process. If we could have the old BOM features back but with new code? Would have been ideal.