The primary reason for the low production numbers was the $800 conversion price which moved it too far out of reach for most consumers. It was too close to the base price for a Cadillac
It's the first time an outside contractor had designed and built a customized Chevrolet model which was later sold as a new car with a full factory warranty. The bodywork was restyled to resemble the 1955-1957 Cadillac Eldorado Biarritz, Seville and Brougham by R. Allender & Company

The primary reason for the low production numbers was the $800 conversion price which moved it too far out of reach for most consumers. It was too close to the base price for a Cadillac, which was the target to beat.

Cadillac introduced their Eldorado Brougham, not in spite or in competition with the El Morocco, but Allender felt the need to create a new El Morocco to emulate the new Brougham.
Allender was a longtime Cadillac owner who envisioned a smaller, easier to maneuver Cadillac that his grandchildren could learn to drive with. He purchased a new Eldorado Biarritz convertible in 1955 and reckoned that with some additional bodywork, the new 1955 Chevrolet could be re-styled to resemble the Eldorado.

Problems with the fiberglass body production for the 1956 cars led to the use of steel for the 1957 models. This required comprehensive metalwork changes, including removing and filling the 1957′s rocket hood spears with steel, and welding on the steel rear tail-fin extensions. The car’s interiors and exterior hardware was set aside for reuse or sale to local collision shops.

The first cars were created in 1956 on Chevrolet platforms and designed to resemble the 1955 and 1956 Cadillac Eldorado. The name ‘El Morocco‘ was from a popular Manhattan night club and had similarities to the name ‘El Dorado’.

The 1956 El Moroco’s featured body panels made of fiberglass. A host of trim parts and designed were borrowed from Willys, Dodge and Kaiser-Frazer to complete the package
http://caretro.com/1957-chevrolet-el-morocco/

Cars were purchased from Detroit’s Don McCoullagh Chevrolet at $50 over cost, and Allender used off-the-shelf parts wherever possible. The 1956 El Morocco included a Kaiser-Frazer horn button for its hood medallion, ’55 Willys dash panels for the door top saddle moldings and '55-'56 Dodge Coronet taillights mounted side-by-side above faux exhaust ports that resembled those used on the real Biarritz. The front bumpers included fiberglass reinforced ‘Dagmars’ made from reversed ’37 Dodge headlight shells and the rear fins were edged with trim supposedly sourced from a 1955 Ford.

How did Allender get to customizing Chevies for resale? He was a resale artist. He started his business career and fortunes by pitching fabrics for sale, he would cut off samples of cloths, then take them around and pitch his sales prices. Getting the contract, he'd go back, purchase at wholesale, and sell retail and pocket some profit, and build his business to the point he bought army surplus parachutes, and sold them back to the army at a huge gouging profit. He got into a lot of trouble for that, had to testify before Congress that he wasn't a crook.
http://www.coachbuilt.com/bui/a/allender/allender.htm
Three El Moroccos were at Dick Clarks "57 Heaven" in Branson Missouri
http://www.bransonworld.com/branson-attractions/detail/branson-57-heaven-at-dick-clark-s-american-bandstand-theater.html