I may be misremembering, but my recollection was my then employer started using Ext version 2 primarily since it had the rich widget ecosystem and also supported IE 4 well. We were writing internal enterprise apps and it was a good fit.

As you can guess, that's been quite awhile ago. My memory is fuzzy on the details to be sure.

I also used extjs 2 and i can assure you it didn't support anything below ie6.