I think there are two major points here, one is the versioning of EXTJS which indicates minor releases but in reality they are major releases. This throws people off thinking it is just a plug-and-work with less bugs deal.
Sencha's definition of major release, 3.x to 4.x is more than a major release, maybe a different product.
The second point is way the framework has evolved, got larger, with more tools around it, thus more complex for smaller shops, thus training needed, thus... you get the point. It is Sencha's business strategy, good one, but ever evolving.
In a large enterprise environment where a lot of money in invested in the applications and tool set, it is very difficult to adapt Sencha's evolution of the framework., especially for existing applications. It is a bit easier for new development.