The advancement of motor and power convert technology is also worth noting. AC motors are a huge upgrade over the old dc motors that used brushes.
But with a conversion vehicle the gearbox comes for free with the donor car, and it's typically more work to integrate a single-speed drive than to just adapt the motor housing and shaft to the transmission bellhousing and flywheel. And you're not paying the early adopter tax that's still present on AC drivetrains.
Also, a big plus for DC motors is that the controllers are simpler. AC motors require a high-power adjustable-frequency inverter.
The only downside of common standard DC motors that I know of is that you have to change the brushes every n years, which I'll gladly take over oil changes. :)
Though, I still tend to gravitate toward the simplest solutions, and in this case, a series-wound brushed motor is about as simple as it gets.
Regular DC motors have heavy and strong magnets (more expensive), but the series-wound motors are just wires (and, FWICT, a bit less efficient).
Regarding brushless, I'm sure they're quieter and require less maintenance, but they still are more complex than the series-wound DC.
I suppose the AC induction motors have their place, but for the DIY crowd, I think many will stick with the basics.