Relationship abuse happens within the wider context of a society that values women less than men, so the system that shores up this context (i.e. patriarchy) is very much at the root of it happens.
Messages are all around us, from cradle to grave that men are entitled to lead, to control, to direct and women are expected to support, to nurture to serve. The fact that some men feel entitled to enforce this through the use of physical, emotional and/or sexual threats or abuse is therefore, not that surprising. The fact that there are few social, economic or legal sanctions for men who commit abuse demonstrates if not outright collusion with male abuse, at least most certainly not universal condemnation of the practice.
I would say it's the same for all situations where one group as greater power, control and status than another group (e.g. race, culture, socio-economic class, disability, sexual orientation, etc.)
If human beings were not valued based on their sex, then there would still be conflicts and still be examples of coercion and abuse in relationships because there are a plethora of factors that can lead to conflicts within human relationships of all types (think about work colleagues, think about neighbours, think about friends who fall out, etc.) But, we wouldn't see that abuse being so gendered if not for entrenched sexual inequality.
Often commentators try to "pin" the reason or reasons for relationship abuse on other factors - all sorts including substance misuse, religious traditions, class traditions, mental ill health, financial worries, etc. While some of these may have an impact on the context and choices people make in relationships, they are not the reasons for abuse, nor would addressing these eliminate the spectre of abuse, so long as gender-based oppression exists.