The answer is really simple. I could had this consideration, but not.
I really would like to do it cross browsers, but the builder runs with MooTools and lot of CSS, JS.
It is only a LAZY question. I do not want to run specials tests. Only that.
I am working with Firefox because it has so many developper tools. I really invite users to install the Web Developper Toolbar of Firefox, and Firebug is awesome as well.
When I did the decision, I choosen also for an Open Source one.
My intention is really not to limit the users, but to concentrate on usefull stuff.
Cook Self Service give you total freedom, no backlinks, no asked license (only authors in headers, please)
But your question is important, I may have asked the same if I was a user.
I think Chrome could be eventually possible (with a non garantee discaimer).
Not my priority at all for the moment.
Firefox is good.
I can also help users with Firebug, and WebDev toolbar, it is more easy and can help with screenshots...
We can also think about creating components for Drupal or Wordpress, But Joomla has been choosen.
Same for jQuery.
Cook and Joomla are full of choices, because it speaks about standards.
Joomla is non-corparated backed, and it is good because the implemented tools an libs are because the bests. Not because of lobbying.
See bootstrap in 3.0 : a Jewel !!
Hope you understand. When all users are using the same it is also more easy.
Thanks for your question.