Monocoat vs. Basecoat/Clearcoat
There are several reasons why automotive OEM topcoat technology is predominately basecoat/clearcoat today.
The topcoats have many functions: provide color/hiding, gloss, durability, weather resistance, etc.
In going from monocoat to two-coat processes, it is easier to engineer the coating system to provide maximum performance properties. When topcoat were all monocoat, it was always a tradeoff between hiding and gloss/protection. The more pigment in the coating, the better the hiding but usually at the expense of gloss. This was not much of an issue with solids colors, but more so with metallic and pearl systems. Monocoat systems are also more prone to color transfer - what you see on your rag after polishing and waxing. The clearcoat alleviates this problem.
The popularity of metallic and other effect colors has especially led to the two-coat sytem. We wouldn't be able to achieve some of the colors and effects popular today in a monocoat system. The special effect pigments in the basecoat have to "orient" themselves a certain way to provide the "flip/flop" effect we all like. By having the basecoat provide color, we can load up more in the basecoat and use the clearcoat for gloss and extra protection over the effect pigments. It is important to have the aluminum pigments "covered" with a resin matrix in order to provide durability.
Air quality regulations have also contributed. For the past fifteen years there has been a greater emphasis on higher solids/lower VOC (solvent) formulations. It is more difficult to apply higher solids coatings and maintain high gloss and DOI (distinctness of image). Many of today's basecoats are now waterborne which helps with lower VOC emissions, enabling the solvent-based clearcoats to remain somewhat lower in solids for appearance.
There are some drawbacks to the BC/CC systems. The clearcoats result in higher gloss and better appearance but make acid rain etching more noticeable. The complexity of an extra coat also makes for somewhat harder and more complex application. Deep scratches down to primer or substrate are still as noticeable, but more shallow scratches only through to the clearcoat are now more visible than in monocoat systems. Of course repairs are also more complex and costly. Well engineered basecoat/carcoat systems also present more challenges as far as compatibility and application.
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