Why " not only is there" instead of " not only there is"?
Citing Sir Charlie Mayfield’s Productivity Commission’s work, the chief economist said “not only is there a long tail of [unproductive] companies, but that many are unaware of that fact.”

Question

Why " not only is there" instead of " not only  there is"?
Citing Sir Charlie Mayfield’s Productivity Commission’s work, the chief economist said “not only is there a long tail of [unproductive] companies, but that many are unaware of that fact.”

Answer

The options are "there is not only" and "not only is there"

This is an example of subject/verb inversion.  It is usually seen in questions but it also occurs in sentences which begin with negative words and expressions e.g. not only, never, hardly, rarely.  It can also be used with "so often, so rarely", etc.  It adds emphasis and formality.

Check unit 100 of "Advanced Grammar in Use" by Hewings.