It is interesting that ExtJs is so much searched for than any other of the most popular frameworks. Specially since you don't hear much about ExtJs lately. AngulaJs though is getting close.
But I agree, extjs needs a considerable amount of investment in time and mindshare, because of the way it is built. I actually like it very much. Hey I built my product based on it.
But lately I have seen extjs as old, and in need of love. AngularJs for example is easily lovable and easy to learn.
But yes, I use it for corporate software, since my company pays the license. Also it's strong in the table and charting parts, which are the butter and bread of many corporate devs.
But I guess it also includes people looking for the Japanese green tea https://en.wikipedia.org/wiki/Sencha
So the keyword still applies. Search for sencha might be mixed with other frameworks they have.
On the other hand, it started to rise in 2007 when ext.js 2.0 was released.
Now these trends focus on a world wide searches. If you look closely at the detail, the number of search for extJs in China, pretty much take the 90% of the searches. And in the U.S. extJs has practically no prescence.
If you look at the detail for AngularJS, most of the searches are in California, New York and India.
So it is really not the best trend analysis, but it does put the power of China into perspective.
Next to Extjs there is AngularJS , well known for it's dependency injection features and it's directives , and after that there is of couse Backbone.
But most of front-end devs dont use frameworks at all , it might not make sense for every project.