said by Teddy Boom:
The wire inside your flat cable is probably stranded and crimping to stranded is harder than crimping to solid conductor.
Not that much harder. You just need a different type of connector.
Crimp connectors for stranded cable have straight "vampire tap" teeth that simply sink through the insulation to make contact with the wires while solid wire crimps have offset teeth that displace insulation on the conductor sides to wedge themselves against the wire.
Using the right connector type for the cable being used makes things much simpler. If you mismatch types, your life may become miserable: vampire taps cannot sink through solid wire so they only make superficial contact with it and insulation displacement on stranded wire may simply squeeze the wire with strands stacking together and slipping right by the teeth without touching them.
As for flat cables and twisted pairs, as LazMan said, there are flat twisted pair cables. The pairs are twisted together and then laid down side by side so the cable only needs to be as thick as individual wire pairs and the coating layer to hold them together. Properly made, those should perform about as good as regular cables. A simple way to reduce crosstalk due to the same pairs always being next to each other is to simply increase the distance between pairs by about one wire's width and continue using the usual different twist rates for each pair.