Conglomerates flourished in the 1960s thanks to low interest rates and a market that fluctuated between bullish and bearish, providing good buyout opportunities for acquiring companies. However, when interest rates began to rise again in the '70s, many of the biggest conglomerates were forced to spin off or sell many of the companies they'd acquired, particularly when they'd only done so to raise more loans and had failed to increase the efficiency of the companies they'd absorbed.
That said, conglomerates can be advantageous, particularly if the conglomerate is well-diversified. For example, Berkshire Hathaway is a conglomerate holding company that has operated very successfully for years.