As well as being flexable the silicon insulation can handle higher temperatures.
This means that you can intentionally use smaller wire as the wire can run at higher temperatures if you can accept a small voltage drop in the wire.
..and the insulation doesnt melt when you solder connections.
Also remember Ebikes use lower voltage and much higher amperage than household 110/220v systems.
High current needs more copper, and that would get unworkable in solid core wire.
8, 10, or 12 G silicon multistrand from HK. ..sized acording to your system current level.
you can get it in most hobby shops also, but much more costly.