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why cant you solder the heating resistors? so they heat back up or something?

You can, but you run the risk of the solder melting during use, most solder will melt around 300F. I have solder connecting resistors to wires. But also I use copper tape to help form the junctions then solder on top of that making a larger blob of solder.

I haven't had a failure with solder, but I have my wire/resistor junctions spaced far enough from the extruder that it doesn't get hot enough to melt.

well i have non lead solder, it melts around 500 but my iron runs at 900 . I should be okay right?

I tried to convert units in my head, I should never do that . Approximately 210C - 250C is where most lead free solder melts.
If you have something that will melt around 500F then you should be OK. As long as you keep the connections from touching the extruder.

thanks will do