They won't do it. And here is why.
1. Have additional hardware to run the different software versions, this costs money.
2. They need additional employees to support the additional hardware and software versions, this costs money.
3. Contrary to popular belief, they have to maintain that outdated software because of integration points which have changed. For instance, the 1.4 client didn't use Battle.net authentication. So as much as people would like to believe it can exist in its own little world, it actually can't. This costs money for more developers to support and retrofit changes to prior versions of the application.
4. The user of said services is required to maintain multiple client installations. A 3.3.3 client can not run on a 1.4 server. Both client and server applications have to be at the same level in order to function correctly. There are major changes to the client software that also occurs in patches which make in incompatible with prior versions (such as the LUA core replacement in the 2.0 patch). This costs Blizzard more money to support the user's different setup.
So lets see here...
Where again is this a decision Blizzard would want to make?
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.