Is there a canon explanation (based on how Star Wars blasters work) for why the blaster bolts are so slow? Intuitively, one would expect them to be like lasers, traveling at the speed of light.
Blaster bolts are made of plasma, not light. See here.
Blasters are a considerable improvement over the archaic laser design. Instead of a coherent beam of light, the blaster fired a compressed, focused, high-energy particle-beam that is very destructive, commonly referred to as a "bolt".
My best reference I can find is: Star Wars: The Complete Visual Dictionary
Originally, blasters were intended to be lasers. For example, in the Episode IV script the opening scene is described as so:
A tiny silver spacecraft, a Rebel Blockade Runner firing lasers from the back of the ship, races through space. It is pursed by a giant Imperial Stardestroyer. Hundreds of deadly laserbolts streak from the Imperial Stardestroyer, causing the main solar fin of the Rebel craft to disintegrate.
"Laser" is used quite extensively in all the scripts, including the more recent prequels. One would expect blaster bolts to travel at the speed of light based on this description, but they only travel ≈100mph. But did you really expect scientific accuracy from the people who thought a parsec is a unit of time?
[Vader] fired and his blasters sent twin beams of plasma into the aft shields.
According to updated canon, therefore, blasters are not lasers which would travel at the speed of light. Since a plasma beam consists of particles it makes sense that blaster bolts travel slower than the speed of light.
Star Wars Wikia has been altered since the new Disney canon changes. However, to clear things up - the Tibanna gas is now the actual element that is fired:
"When the blaster was fired, a small amount of gas moved from the cartridge" -http://starwars.wikia.com/wiki/Blaster
"Tibanna gas produced four times its normal energy output when cohesive light passed through it." -http://starwars.wikia.com/wiki/Tibanna
The gas particles undergo excitement just like laser physics: "An XCiter was a component used in blasters to agitate gas molecules." -http://starwars.wikia.com/wiki/XCiter
The particles are then accelerated just like normal particle weapon physics: "Once blaster gas had been energized it went through this module and was electromagnetically accelerated" -http://starwars.wikia.com/wiki/Actuating_module
As in the question - they don't achieve velocities near the speed of light, so although the energy in the particles may be high, they lack the destructive kinetic energy, according to what's seen on screen.
Because, Star Wars blasters do not fire laser bolts of light.
They fire particle energy beams of gas, Tibanna gas to be precise.
The clones use a kind of plasma bolt which is even more effective against droids, but most others use particle beam weapons.
A link to the blaster article on the Star Wars wikia.