The common thought is that they are "Separated by a Common Heritage."
Sounds about right.
Original reasons for the split are mostly obsolete today. If I can remember right, the more traditional (CRC) members objected to singing hymns instead of just Psalms, allowing non-members in participating in the sacraments, and membership in Masonic Lodges. They were also not fond of the shift away from catechism.
More recently, the biggest sticking points have been the role of women in the church, private v. public schooling, and financials... with financials probably being the biggest hurdle for those still hoping for reunification. I think the theological differences are small and could be overcame fairly easily. Fiscally, the differences are much bigger. CRC member churches pay significantly more back to the denomination (for a church of 100 members, roughly 5 times more). That money funds a multitude of offshoot missions that don't exist in the RCA like ReFrame, Resonate, The Banner, etc... However, 20% of those dollars also go to Calvin University and Calvin Seminary. The RCA approved this past weekend to change Western Theological Seminary from a RCA seminary to a "officially related" institution (the same status it already holds with Hope College). My point... 0% of RCA mission share dollars go to either institution.
Members not interested in reunification would argue the CRC has a history of not being the best stewards of those funds and/or not a fan of the fact that 20% of those dollars go directly to either Calvin University or Calvin Seminary. Even if they moved past Calvin, they tend to argue that the bulk of their tithing should stay local and be managed by their church, not the denomination.
CRC members uninterested in reunification typically view the RCA as less unified than they'd prefer. They have great pride in CRC's global outreach programs and tend to dislike the move away from CRC traditions towards more "non-denominational" faith. This is often paired with the feeling that the RCA has become too complacent in allowing more secular and liberal thought.
