Speaking as an enterprise customer on long upgrade cycles, RIM/Blackberry badly judged the situation vis a vis their enterprise customer base.

The new Z10 was not compatible with their large install base of Blackberry Enterprise Server. Anyone still running BES at the time the Z10 was introduced was likely to be a shop interested in security and control of the mobile platform their end users were using.

Which is why businesses like my employer and many others kept buying BB OS7 devices (did you notice they didnt break numbers out and actually mentioned BBOS7 as a significant portion during their investor call ?)

TL;DR: They thought all their enterprise customers would upgrade fast, they guessed wrong.