This is the only area of infidelity that I feel I have a handle on and probably the one area that sticks in our hearts and minds the longest.
The tough answer is the why will never be a good enough reason. They why is a symptom of a damaged and immature person making selfish choices. So the why boils down too they talked themselves into making a choice for short term gains and decided the consequences were worth the risk.
I have yet to meet a betrayed spouse the listened to the why and went "Oh ok that makes sense, all's forgiven."
The thing is nothing MADE them do it, cheating is always, let me repeat, ALWAYS a choice on their part. It doesn't matter if you are the most obnoxious, rude, demanding, nagging spouse who ignores them, withholds sex and has totally "let yourself" go. The cheating spouse has chosen to betray instead of dealing with the unhealthy elements of the relationship. Now they might use all of that and so much more as their excuse for the choice but it is still their choice.
My husbands choice fully laid the blame at my feet in the guise of being "helpful". I have a chronic illness. This makes my energy levels low, my libido low and often in too much pain to get "frisky" as often as he'd like. Now lets be clear my husband wants to have sex twice a day, everyday. That is more than I am actually physically capable of doing. When I asked him why he cheated, he said "I thought it would help you, it would take the pressure off you since you're sick!"
What the actual fuck! So my illness became his excuse to do something he wanted to do in the first place. This was a big light-bulb for me. The why means nothing, the why will never satisfy, the why is only a symptom. So do your best to let go of the why it is not going to give the answer you need. What you really need to figure out is, what is the symptom the why is pointing at and is that something you want to change or repair. if it is good luck, it's going to be touch but hopefully worth it. If you don't want to change it then happy trails and remember it wan't your fault, you didn't make them cheat!