They are talking about why acquire rather than do an IPO - which is a completely different question to "why acquire the remainder at all".
An acquisition, when done for cash or debt, results in an overall increase in existing shareholder 'value'. Where as purchasing something by issuing new shares and diluting existing shareholders, is completely different. By acquiring the remainder 45% by issuing 45% (or there abouts) new shares, then existing sharehholders are in a worse place because they now still own the same amount of the entity but also need to share profits with the new shares on existing businesses they own.
