[MarsAscendant]

I suspect it has to do with connotations.

When you hear "heavy" as used to describe something, you automatically assume it to mean negative. "Half is heavy" has a more negative connotative meaning than "Twice as light", even though they have the same denotative meaning. In other words, they effectively mean the same thing, but represent that thing differently.

I also suspect it has to do with the positive/negative framing. Ideally, you want to leave a positive impression on your audience. If you're talking about scientific discoveries – especially something as cool as a material that's comparable to titanium in terms of strength, yet is much lighter – I reckon you want your audience to feel excited and inspired. In this case, using positive framing is reasonable and conducive to the goal.

I'm not sure how appropriate it is for a popular-science publication to refer to framing when talking about new discoveries, and am not offering support for or against such usage in this context. The decision to put the phrase in such a way, however, may well be an informed choice that makes sense from the perspective of the goal of such a publication.