Why….because their claim did not hold water and agreed if money was available the max they would receive is 72 million….however class 19 received nothing…..unless they were special like AZ and received a distribution as well
Only the Underwriters can give you a precise answer to your question. Anyone else's answer is simply going to be a guess. So, how it looked +10 years ago the Underwriters had a $28 million Class 18 claim and a $72 million Class 20 claim. With the eventual approval of POR7, the Underwriters negotiated to drop the Class 18 claim while retaining their $72 million Class 20 claim, (their Class 20 claim then became a Class 19 claim). With their Class 19 claim they received WMIH Common Shares, something they wouldn't have received having their Class 18 claim.
Keep in mind that the belief +10 years ago was that the waterfall wasn't going to reach any further than Class 16 (Piers), with Class 16 being left impaired. (not even being paid in full.) So, even if the Underwriters had negotiated to retain their $28 million Class 18 and give up their Class 20 claim there still was no guarantee that their Class 18 claim was to be paid in full, much less to see any payment at all. That's how it looked +10 years ago.
Then please explain why the underwriters chose to give up their position in 18 for holdings in 19.