Many reasons.
The most common is that, after about two years, many couples know each other's most reliable routes to satisfaction. They repeat the template that works best, and that familiarity and predictability begins to erode the excitement.
When they get into daily domestic routines that increases the same effect. If they start to take each other for granted and not show appreciation, genuine warmth, good will and affection, that begins to tip the relationship into the negative.
If they have kids, that reduces privacy and increases stress and exhaustion, which is a major turn off. It starts to take effort to organise a date and time alone. Some partners fail or forget how necessary that is.
If they start to quarrel and don't learn good communication and conflict resolution skills, the ill feeling will kill their desire for each other. Most people don't learn how to actively listen with empathy either at home or in school, and it's not visible in culture. So the only way to learn is to seek it out and become proactive with daily practise.
Some couples have unequal libido. That's a bad match.
Not all people are raised with monogamy as their moral or ethic - some are polyamorous, polygamous, polyandrous - or just bohemian and free.
It's important not to assume someone's values are the same when dating a new person. If you like them and are thinking of starting something, ask the important questions before getting involved. That includes their relationship and sexual history.
Some situations are exceptional. If a partner becomes ill or incapable of sex, perhaps needing full-time care, it would be cruel to expect the carer-partner to be celibate.
Long distance relationships.
Frequent long term absences, for instance, for work.
Some people are born with less oxytocin - they can't bond well. They stray easily and can't form solid long term relationships. One sign is that they show zero jealousy.
