Because it is very unbelievable that you had to get up to 50+ mph to keep up with the flow of traffic. I have never experienced speeds anywhere close to 50 mph through many years of using Rock Creek Parkway on my commute.

Cheaper? Definitely. Much easier? It depends on where you're coming from. I live in Adams Morgan, and I wouldn't call metro the easiest way for me to get to DCA. It would take about twice as long as taking an Uber (my usual way of getting there) or driving. I usually only take the metro to DCA when I have time to kill, don't have a lot of luggage to schlep to the metro station on foot, and the yellow line is running to Columbia Heights so I don't have to transfer.