Simples: They just don't need to. They still have a commanding market share towering over Firefox and the speed at which they're losing market share dictates how swiftly they develop their own product. Firefox is over 5 years old now and whilst it has gained a good market share, IE is still the daddy by a long mark, and will continue to be for a long time yet.
There are plenty of office intranets and corporate/industrial systems based on IE that will keep its market share high for years to come.
JIT will come to IE eventually, but they're not in a panic to roll it out like Opera were because they don't need to. They're not as fast as Opera or Chrome, nor support as many standards, but their product works and they make money. They'll continue to hack away at IE over the next few years and manage to complete what Opera, Apple or Google do in a year, but that's fine by them.
You can be sure that if their market share suddenly dropped and corporations started developing their next generation intranets and control centres for Opera, Chrome or Firefox, they'd soon get into the same gear as those companies and we'd probably get a major new version every year with massive improvements in performance and features.
Now, cue a Firefox retard to start foaming at the mouth and compare the combined market share of all versions of Firefox ever made with one specific version of IE, possibly even one that doesn't even have the largest slice of the IE pie. IE6 lives folks! And 7! And 8! And soon 9! It's alive! Aliiiive!