Server architecture has been strongly shaped by the realities of technological development. Since the beginning of the 80s, the introduction of the microprocessor and the pace of (particularly) semiconductor technology development has transformed a business once based on the value-add from proprietary systems (solutions developed by one manufacturer just for its own product line) to a market built upon commodity components (components and subsystems used in the PC industry).
As the transformation unfolded, mainframe architectures saw their market share stagnate or even decrease. Minicomputers disappeared completely, replaced by servers based on standard microprocessors. That said, there has in recent years been a notable stabilization of the mainframe market (sufficient for some analysts to pontificate that "the mainframe is back"). Several factors have played into this phenomenon: reduction in mainframe costs because of the adoption of CMOS technology; the use of peripherals from the UNIX and Windows worlds; the acceptance of lower margins by manufacturers to compete with high-end open systems; the basic robustness and maturity of the mainframe systems; the move to server concentration that replaces distributed systems around a company with a smaller number of single-site...
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