A lot of it is the second coming of the weight weenies. Everything's become minimalist, in an attempt to get weight down below what used to be the norm for BMX race bikes. The rims themselves have gotten stronger, over the years, partially offsetting the strength lost with the spokes.
Running brakeless has nothing to do with weight, though. That fad started in the 90s, before everyone started worrying having the lightest bike. A lot of people say it lets them ride smoother.