Its probably a combination of factors. The FT article cites political risk (regulatory change over a long period is likely coming), the recent £123m fine that was imposed (potential for big fines is going to spook any investor), and difficulty getting the agreement of all current stakeholders.
Can't blame them really. Imagine coming in as a new equity investor to a company holding billions in debt and needing billions more capital immediately, which is likely to be incurring regulatory performance fines, an imminent CMA appeal, constrained or no dividend returns, and unable to secure any regulatory/government guarantees to mitigate these risks...would run a mile and they have.
Proves they have done their due diligence at least, something the previous investors who Macquarie sold onto didn't and they lost their investment because of it.