Why did ford create the crown Vic & grand Marquis they're basically the same car?

Ford was trying to sell an upmarket version. Car manufacturers often sold the same type of car under a number of different brands and with minor variations. It was a marketing strategy.

Mechanically and to a large extent cosmetically. But the materials used in the interior and details of the trims was of a higher grade material or at least perceived as superior goods. It is not like this was new with the Crown Vic and Grand Marquis. The Ford Falcon chassis for example was sold as several different cars- the Falcon, Comet, Fairlane, Mustang and Cougar were all the same chassis with small differences in details. Even in the 50's the Mercury  line and Ford line were the same car with different details.

The reason was to lower product development and production costs, while providing a unique product for different social economic groups.

The Grand Marquis is the upscale version of the Crown Vic. Marketing targeted toward those who think the Grand Marquis is a fancier car (cosmetically it is).

EVERY car maker does that.

Yeah the crown Vic was intended as a high-performance law enforcement vehicle and there are the passenger models but the Grand Marquis is basically the same vehicle it's just more luxurious. It gives you three different ways to ride in a police car!

It was all marketing and using most of the same parts.

Sales. Their target audience they market to are 

F %C^ing

O bviously

R etarded

D rivers.

That company even circles the problem right in front of the driver between their hands and they some how miss it or look past it.

This is done all the time in the Auto Industry same platform for different car levels - GM is famous for it - (cavalier and Cadillac cimmaron. Toyota and Lexus (LS 300 and a Camry in the 90s were basically the same car). 

No need to re-invent the wheel when marketing can brainwash the masses. 

The Grand Marquis is a Mercury and it was common for Ford as well as General Motors to make very similar if not identical cars and sell them as different names under their different brands. 

Because one was a Ford brand of car and the other was a Lincoln Mercury brand of car.

Same company, but Lincolns are marketed to more upscale clients.