Well, I guess there is a case for supporting old browsers. But the practical side of it is we have over 20,000 lines of official javascript code (and a lot more from third party plugins) that would need to be carefully reviewed alongside compatibility tables for all the old IE versions, and hacks and workarounds added as necessary. I would not want to sell a broken product to schools and I don't think you fully appreciate the work needed to test and verify all features work properly with old, non-standards-compliant browsers.
For example, does 'Fullscreen in browser' work? Does every combination of project and layout setting work? Does WebStorage work prior to IE8? (It doesn't.) Does XML parsing work properly? Do the form controls work? etc. etc.
It is easy to wave away broken features. But suppose a school somewhere is using IE7 and wants to use WebStorage to teach client side storage vs. server side storage (as a random example). Oops, it doesn't work... they send us a support request, and if we officially support it, we're obliged to fix it. That could be a lot of work re-writing a plugin for an old browser when we're halfway through adding support for the latest features being added to browsers. Again, I predict there are countless issues where there will be technical issues like that. It's difficult for us to spare the resources to either fully test our entire engine on IE6-8, or to deal with support requests as they come up.
Again, browser upgrades are free and should be relatively straightforward, and can be installed side-by-side with IE as a non-default browser so exact compatibility with their version of IE is preserved. I am finding it difficult to understand why this is such a huge problem for schools. If they are going to the bother of installing Construct 2 over computers site-wide, why not just bundle it with Chrome at the same time? They're already doing the work... then everything will work much better with C2 as well.
By all means use FlashCanvas if it works for your needs, but us supporting it officially is a thorny issue that we'd prefer to avoid. We'd rather say: this software requires a modern browser.