Changing succession laws was grounds for a re-approval of a personal union, because it was changing the terms; and -
The conservative faction of "Germans" would not be down for altering the Salic law. Even though the Holy Roman Empire had been put down, Austria and Braunschweig both meddled in Hanoverian affairs, and the constitution of 1833 was already seen as insidious infiltration of British ideas of "liberty" over continental absolutism. Pressing the issue would likely lead to a civil war.
Sleswig - Germans were concerned with putting up a unified front of opinion against Denmark, which also had more generous succession laws, and wanted to absorb Sleswig, while the Germans wanted to see Sleswig and Holstein become an independent part of their confederation. Allowing Hanover to succumb to a similar British scheme would have angered Prussia, who, as we see in 1866, could defeat it militarily.