why some car use disc on front and drum on rear wheel?

Because drum brakes are cheaper to manufacture. So a lot of cars use them on the rear because it saves the manufacturers money so they can increase their profit-gouging on consumers and because rear brakes dont provide most of the braking force anyway

IT is CHEAPER to use DRUM BRAKES that are BULNERABLE tostreet water and stop working right! I  have NO OTHER explanation! It is PRICIER to use DISK BRAKES fro some reason! DRUM BAKERS are VERY inefficeint and older gm X cars evenused ALUMINUM BRAKE DRUMS that FALL apart under HEAT! Teh BEST cars wil ALWAYS have FOUR  WHEEL DISK BRAKES and FOUR WHEEL abs brake pumps! ALL CARS need the BRAEK FLUID power flushed and CHANGFED every 3-4 yearas or HOSES and LINES wil rot from the INSIDE OUT! BROWN fludi is a DEAD fluid and wil begin to RUIN BRAKE SEALS int eh MASTER and inthe ABS PUMP!

I don’t think it’s quite as simple as drums are cheaper than discs. What it is , is it’s much easier ( and so less complicated and so cheaper) to make an efficient handbrake using drums than it is with discs.

The main reason being when they cool down, the drum will tighten but the disc will loosen.

Costs. It all boils down to costs....even if rotors are more efficient than drums. Most people dont care rear disk rotors, they just like how the car looks overall inside or out. In the old days, cars had front drum brakes. 

Because it's the cheapest model they make.

Most vehicles distribute brake force 70/30.  That's 70% to the front and 30% to the rear, rear drums work well under the 70/30 rule.  

try on ebay or junkyard