This has been a question bandied about in various forums, asked of me, and one that people seem to have pretty strong opinions about. While there are a variety of positions available, let's just take a look at a few points.
On the one hand, within mono relationships, there are pressures to satisfy most/all the needs of one's partner, and not usually as many resources for personal growth. There's also a lot of societal reinforcement in the positive direction to be part of a "traditional" monogamous setup. Perceived safety is part of the package, predictability, perhaps even ownership. It's supported by the law of the land, practiced (poorly or well) by the majority of the population in this country, even within less mainstream communities, like GLBTQ, and touted by the majority of mainstream religions, most mental health professionals, and Hollywood as the way to go.
Really, the question that we may want to consider is why _isn't_ poly harder than mono? While things can be more complex within poly relationships, it is often purely because there are more factors (people) to track and more opportunities for miscommunication to occur. Here we have a choice in living and loving that receives next to zero support from society at large, if not active antipathy, and yet more and more people are moving this direction, coming out, writing, talking, and sharing their experiences, thoughts, and feelings about open relationship structures!
So why do poly relationships end? Mostly for the same reasons that mono relationships end. For me, as I go through the process of changing the way that PG and I relate to each other, I am very grateful for the skills, relationships and community available to me because I live and love poly. There are skills that, while I may have learned them eventually in a mono relationship, I've had much more opportunity to practice and grow with as a poly person.
Poly is like serving concurrent sentences, minus the orange jumpsuits, plus lots of consensual sex: In my 34 years, I've spent 27 years in relationships of a significant nature. No, I haven't been dating since I was 7, but I have been privileged to share my life with others for that long all together. It might not be directly equivalent, but it's certainly more relationship time/skills/experience than I would have gotten as a mono person by this age, no matter how aggressively I put time and energy into that single relationship.
Here is why I still think it's "easier" to be poly: When something comes up, I have the tools in the toolbox to work it through, even if it is darned complex. There are times where I look at a situation and feel a period of despair, that it's too much to handle, too hard, and then a little piece falls into place, and a lover shares a piece of wisdom with me that helps something else become clearer, maybe I am able to identify an underlying issue, then draw a parallel to a previous set of circumstances, and pretty soon the snowball is rolling down the hill without me pushing it the whole way. I'm just flat out better resourced than I would be otherwise.
So, even with what can feel like the whole world telling me I'm crazy to being doing something so very "hard", I am happy to be poly, because, for me, it's the gift to myself that keeps on giving.