The cases you cited had aggravating circumstances, though, such as drugs, domestic violence, etc. Elizabeth Case, for example, had a history of drug abuse. She was out on bond for robbery and theft charges, fraudulent use of a credit card, and was dumpster diving when she left her son in the car. DCS had an open investigation on Dylan Levesque for drugs, child endangerment, and attempting to strangle a woman.
It was a poor choice of words on my part, however. I should have said that Ross Harris is the only person sentenced to LWOP for cheating on his wife.