The kind of MMORPG market which this thread relates to is just favourable towards MCCode and GRPG architecture, for various reasons.
One of the main reason is when a game gets deployed on an engine like MCCode v2, if the gameplay process is kept un-touch; talking about the main menu, explore/city, functionalities offered inside of Explore/City, Travel, Buying items, Attack etc, then it is very similar to many other games out there (Regardless of the game being different genre, crimesrpg, sci-fi and so on). Most users have experienced this UI/UX functionalities already, and most enjoy it (quite evidently otherwise we won't be here today). Users who haven't experienced this sort of gameplay before (newbies), they are still able to enjoyably and easily learn the gameplay in order to get addicted (the whole point for games like these).
MMOs are growing very fast on mobile devices with the right UI/UX (clean and responsive) currently IMO. Gameplay which matches the old retro games like TornCity, MonoCountry, HoboWars, CriminalCountry and few other popular games of the past. Players demand is growing again. It had died out for a while imho.
Developers point of view on the popularity of MCCode and GRPG engine is very much similar to the gameplay produced by these engines. Vast amount of support and resources out there that keeps it very consistent on how MCCode engine's codebase and front end functionalities work. Some may say it's because of the simplicity and no following of any development process. I agree. Hence it's very easy to learn and expand all in basically 1 directory of source. null official documentation out there, it's community driven backed by simplicity.
This is highly subjective to an individual developers programming level. Highly experienced and disciplined programmers will find it rubbish to use these engines. However, the market for these Software's facilitate developers to first learn the architecture of a normal MCCode game before they start having fun building new functionalities in the game. Learning and applying what you learnt is a reason for this popularity level.
I still look forward to see a popular MCCode style gameplay running on iOS and Android apps. C'mon someone!