Kingdra is going to be active most of the time, and almost no one plays Bubble Coat. It's next to impossible to search for, it only works once, and it only helps against one specific type of Pokemon (the one your main attacker is weak to), so it generally gets left in binders.
Oh and I forgot to mention that the ability to KO any subsequent Claydol on command that pops up after you Voltage Shoot the first one is another way that Raichu can get the edge against Kingdra. They'd still have an Uxie or two to help refill their hand perhaps, but eventually Dragon Pump is going to be ineffective without constant Cosmic Power support and that means Raichus can stick around longer.