The preference for lahd draws on reported early practice and the wording اللَّحْدُ لَنَا وَالشَّقُّ لِغَيْرِنَا
Translation: Lahd is for us, and shaqq is for others.
Arguments permitting or preferring shaqq consider transmitted practice and circumstances such as unstable soil. This is primarily a dispute of preferred form, not respect for the deceased.