SlimPort uses defined micro-USB pins for DisplyPort signals. MHL 1 and 2 are a competitors to DisplayPort and are not connector specific, but micro-USB was the most popular. Because it does not have a defined pin out profile, not all micro-USB MHL adapters work for all MHL capable smartphones. DisplayPort over USB-C uses a defined pinout profile as part of the DisplayPort USB-C Alternate Mode profile. Thunderbolt 3, another Alternate Mode profile of USB-C, also includes DisplayPort. BTW, there is also a MHL 3.0 Alternate Mode for USB-C, but I don't know of anyone who is using it.
Basically, think of DisplayPort over USB-C to be the direct follow on to SlimPort over micro-USB. Both were developed by VESA.