Publicity. Simple as that. It's news-worthy article (as you can see here) and they gain users or at least a good name from it. Chrome's campaign value is in billions of dollars. Why does it surprises anyone they want to spend some more?

There's very little good information on the relative security afforded by different browsers. In that vacuum we've seen tremendous amounts of misinformation spread (both intentionally and accidentally). So, in order to get a better picture we decided to commission an objective study that would be transparent, repeatable, and verifiable.

A good study requires knowledgeable experts (who typically don't work for free). So, we sought out a respected, independent security company with a team of very well known researchers. They set the terms of the study and put their names and reputations behind the results. All the data and tools are public and open to independent verification. If anything isn't above board you should already have whatever you need to prove it.

So, I think there's an incorrect assumption in your original question. Browser makers don't always put out studies like this. To my knowledge, no browser maker has ever commissioned a study remotely like this in terms of both scope and quality.