Well, I guess that they had to choose
Internal features vs External Features.
I was very happy with MX5 and all its features , and I didn't need anything else. But I saw the community asking a number of times for support of Chrome extensions.
So that's where Maxthon put the focus on. Now if you support an extension for translation it doesn't make sense to also develop an internal one.
I agree with you , I liked more the internal translation tool MX5 had, but the Google Translator plugin is not that bad... and I understand they don't take the effort to do it again.