Why do you disconnect the negative first in car battery?

You should be a bit more specific. Negative what, and where? It’s possible you’re talking about a car battery.

If that’s the case, it’s because the car’s body is connected to negative (by convention, not necessity; a few cars have been made with positive earth (ground in the US), so you need to check this). While you disconnect a terminal your spanner may contact part of the bodywork. If you’re disconnecting the positive while the negative is still connected, this will cause a short circuit; a very strong current will flow, possibly causing the insulation of the negative cable to catch fire, or the sparks may ignite any fuel fumes.

If your spanner doesn’t touch any part of the body, it doesn’t matter which terminal you disconnect first, but realistically it’s almost impossible to ensure there can’t be contact.

Also, when reconnecting, do negative last. In general, don’t do anything to positive, or to the positive side of anything electrical in the car, while the negative battery terminal is connected.