I think the simple answer to your question, because it can always be better or you can always get more.
Now, I hold a very unpopular view here that a lot of men are cheating because of sex, they feel (legit or not) starved for sex in their relationship and cheating is an easy way to fix that and keep the relationship they really want (their marriage). My W's AP fell into this category, when I spoke with the OBS, she said "we have sex at LEAST a few times a month, he gets plenty". Sigh. He had as much sex as you have with him in a month in a single day with my W. He was, again, perhaps entirely his own doing, "sex starved" and sought out an A to fix that. People don't like this explanation, but it's as direct an A-B as you can get, sex is the one thing on offer in most affairs that's not available from a friendship and yes, IMHO, it's the reason that a lot of people pursue them.
So, why? Well, one explanation is above, "sex starved". For me personally, sex with someone new is often very exciting and I think that certainly describes a lot of cheats I know, they just enjoy it (the sex) and want to continue to recreate the experience. There's the whole madonna/whore thing, he may have wanted to do things he was either embarrassed to bring to you or he knew you didn't want to do, so acted it out in the A.
I think a lot of this comes down to personal difference, for example, where you say "never occurred to me to want to sample someone else" that statement just doesn't resonate with me at all. I'm a BS, and I've never cheated but I completely understand the allure of it; yes, "sampling" someone else sounds like something I'd enjoy, and I think a lot of it, particularly for sexually motivated cheats, comes down to just that, the curiosity and desire for something new.
Now, if you've made it this far, please let me turn this around, while I can give you lots of "reasons" why he might have or why I can see his point of view, NONE OF THAT MATTERS. What matters is he promised he wouldn't and then did. Sure, he might have had, what in his mind seemed to be a "good reason", but guess what? I can generate 100 "good reasons" (top of the list, you cheated on me first!) to go have an affair and yet, somehow, I don't and haven't. Because I promised I wouldn't, and I intend to keep that promise. My W was very asexual in our relationship before the A, we did not have an "amazing" sex life we had a mediocre one, I still didn't cheat. So when people tell you "It wasn't you", in most cases, I think that's true, it wasn't you, it was him. Having a mediocre sex life isn't license to cheat, if it was, I would have been out years ago "sampling" other people.
You weren't enough because he didn't make you enough. It's that simple. People here have said I'm one of the most sexually motivated men they've ever heard from (I prefer "most honest" but, whatever) but I made my wife enough and didn't cheat. Yes, I do think, in many cases and perhaps yours, the WS is cheating just to get their rocks off, but that's not because you're not good enough, it's because they can't (actually, WON'T, they can, but don't) control themselves.