Why has the world not embraced open handed playing? Two reasons... #1 - the old cats out there who teach closed because that's how it's been done and #2 - it's a bit difficult to be good at open handed playing and teaching a young kid to listen for equality of sound. Have you done that? Well, you try that.
How to improve playing open handed is simple... practice things with both hands. If you're playing a groove with your right hand leading, switch to your left and listen to what you're playing (and record yourself, too). You should be listening for rhythmic accuracy, dynamic accuracy and the same feel. It's really easy to play the same notes open handed. It's more difficult to capture the same feel. Simon Phillips would play grooves on his drum pad with both leads and really attempt to emulate the feel of one to the other.
It's also worth noting that despite how much practice we might put in, we're never going to be 100% equal in our hands, but we should strive for perfection nonetheless. Sure, we can get close, but the musculature and neural pathways won't be the same for each hand. Play rudiments and excerpts on a pad with both hands leading. I'm a fan of 1-2-3-4-3-2-1. Singles, Doubles, Triples, Quadruples, Triples, Doubles, Singles - lead with both hands and listen for equality of sound and rhythm. Also, don't be afraid of the "backwards" fills - going up the toms instead of down.
Going from closed to open also requires a significant re-wiring of your brain. You're so locked into one system and now you're trying to change it around, so not only do you have to rewire, you have to reroute and install new cable, so it will be a long process. It can be kind of like playing with a handicap and you may have to think diffierently to get comfortable. I'm one of those guys who switched over to "partial open" and while I can let my left hand go on autopilot, I'm still thinking in terms of right hand and how both hands work together. Now, if you asked me, I could execute YYZ with my ride and hat on my left, but I'm not comfortable yet (after years of playing switch) to make the full commitment.
I promise to be more coherent later.