Because Chrome has the backing of Google behind it, which has in turn made it the biggest browser in the market. Maxthon isn't, and won't be, the next Chrome.
Because of that, forcing developers to create content that only a minute number of users will be using isn't viable. Which is why you don't see many of the big addons on the other browsers on Maxthon.
While it's great that there are Maxthon specific API's, if a dev isn't willing to put the time in to use them it's not really a feature to talk up.
While not a feature, the Youtube Centre addon for Maxthon never had the icon display correctly. All that was required was to copy the icon files to the correct folder in the addon. But even that was too hard for the dev. So to add other specific API's is a bit of a stretch.
Maxthon still doesn't have something that will draw users to allow for a more viable addon platform.