Why the rally? This is a tricky one to call, because volume is average (it's been weak for a month) and volatility isn't spiking, either up or down. Instead of one answer, this is one of those days where several factors have combined to create a rally:
Bottom line: there has not been a lot of movement in the markets because there haven't been a lot of reasons to sell. It's hard to make a case for a big drop unless you believe: 1) France is pulling out of the European Union; 2) the tax cut program Trump has been pushing is dead; or 3) the U.S. economic data continues to worsen into the second and third quarter, and bond yields keep moving down.
For the moment, the markets seem to be saying that these prospects are unlikely.