Why did the GHQ not promote English during the occupation?

You can't "abolish" a language by decree. People can't just be ordered to forget how to speak, nor can they learn an entirely new language within a short period of time. As long as the language remains alive, Japan would simply revoke English the moment it regains sovereignty - as it did with numerous GHQ initiatives on in 1952. Thus, the only way to implement your idea would be to occupy Japan for decades until the language cease to be self-sustaining.

A semi-permanent occupation like that was obviously never going to happen. In which case, even setting aside the ethical and moral implications of such a move, it would be completely pointless to even try. All that it could possibly accomplish is antagonise the Japanese, potentially making life difficult for the occupying force, and harming future geopolitical interests.

For reference, the Nationalist spent four decades imposing Mandarin Chinese on Taiwan after being driven there in 1949. While regarded as one of the most successful examples in contemporary times - Mandarin has nearly totally displaced Hakka and Hokkien in everyday use among the younger generations, as well as in most social interactions in general - Taiwanese remains widely spoken by older people in a private context by the time the policy ended in 1988.

Hong Kong under occupation by UK. Or when Japan invaded South Korea

Both of these examples refer to colonies, not a temporary occupation. Multi-decade projects are feasible when you don't ever plan on giving up control of a territory. Britain in particular gained possession of Hong Kong at a time when there was less than 10,000 native inhabitants; administration could by run entirely by the English and in English.

Nonetheless, in neither case did the colonial power outright "abolish" the native language.

Some say it is because of high literacy among Japanese and because GHQ are "impressed" with it.

That's not quite what happened. At the time of Japan's surrender, there was a belief among some that an uneducated population enables militarism. This was an obviously flawed theory given what had just happened in Germany; nonetheless the perceived complexity of the Japanese language gave the idea some plausibility.

This led to a proposal to replace the Japanese writing system with a Latin script, hence the context for the 1948 literacy survey, which purportedly revealed that only 1.7% of the Japanese population were "illiterate" (for some definition thereof). While the methodology was flawed and the results less than reliable, it was nevertheless enough that:

"The CIE was astonished by the survey results, admired the excellent education in Japan, and went back to America without loosing a further word on a Latin script reform." _{- Heinrich, Patrick, and Christian Galan, eds. Language Life in Japan: Transformations and Prospects. Routledge, 2010.}

Note, however, that this was about writing Japanese in Latin alphabets; not about abolishing Japanese altogether. Hence why they investigated literacy.

it might be reasonable to force the use of English, which would help USA gain more advantages in the future (academically, economically, etc...).

Forcing people to use a different language is an inherently unreasonable idea. Moreover, the idea that it would be advantageous for the US is highly dubious. People can be bilingual. Those doing business internationally can learn a second language or hire translators. These are not insurmountable barriers to either academic exchange or trade.In our timeline, Japan and the US became close allies with intense trade links despite the language difference.

Above all, your hypothesis requires that the imposition of a foreign language does not embitter the Japanese people, which is not at all a given.