1) Because they are Enterprises.
2) Because nobody wants to support those browsers.
If you think it's a declining market that'll be dead in 18 months, we have contracts to support IE 6 until 2019, not to mention 7 and 8.
I can't remember how high it was, but let's say it was around 10%. We were told that we had to support all the way down to IE6. To do so, we raised our prices, almost constantly during that project, and they happily paid it. At that level, an agency can change thousands for legacy browser support, and the client will happily pay, because in their mind that 10% will spend far more money than what we can charge.
With that in mind, that's how I view browser specifications today. For clients with an existing website, I check what percentage of their users use legacy browsers, and work out with them whether the cost of supporting these users against the potential benefit. A lot of developers and agencies simply won't do this because they either can't or won't find a solution for legacy browsers.
There's a ton of money there, but you need to build the right organization. Some companies will find it easier to sell to smaller businesses.
Once your competitions builds a better mousetrap, sends in their sales team then explains that all your customers need to do is install Chrome, it might get a little ugly.
Fully supporting IE 6 doesn't constrain you at all with the modern browsers, we take full advantage of the recent stuff. We basically have 2 codebases wrapped up behind a single API.
No, since these are his customers exactly because they don't want to install Chrome.