Why have human resources aspects of mergers & acqusitions been ignored
or handled poorly in the past?

Having worked with HR for a number of years, I believe that it has
been impossible for the HR Director/ Manager to bring sufficient casue
or reason to the board to include this for consideration. The main
reason for this is that for many years it has been impossible to
easily quantify the results of any such report, even though it has
been shown that a high percentage of these transactions fail because
of the inability of the management team to bring two opposing cultures
together once the transaction has taken place. Most mergers and
aquistions happen when two companies are competing for top place, and
it follows that the top directors are also doing the same. The real
reason for including the HR team would be to ensure that the right
people are in the right job once the transaction is complete, and for
that you would need to include competence measurement and personal
profiles.
It is only recently that companies are really starting to take on
board the value of competence measurement, and for many it is still
proving a daunting task as there are so many levels of competence to
be considered.
Once the board of sirectors really understand the value of this type
of measurement, I believe that there will be major role for the HR
team to play in this activity.

One of the planned consequences of mergers and acquisitions is to
combine and streamline some levels of staff  - firing people.  No
matter how well it is done (good outplacement, retraining,
consultation, office facilities for people while job-hunting, etc.),
it is onerous and upsets the persons involved.  Business school
journals may praise the handling, but those affected will still
complain in the community.

There are "better" ways of handling the HR aspects, but none that will
be satisfactory in the eyes and minds of those fired  - maybe except
for those who end up with better jobs.