War Communism was a brief state of affairs during the Civil War period, and the NEP was introduced while essentially capitalist economic relations remained very much in recent memory. Rather than a wholesale leap into a market economy based on rapid privatisations of what had always previously been state-controlled industries, it was a partial rollback of the highly coercive central control introduced during wartime, to the market relations of previous years, but with the Soviet authorities retaining ultimate control. For example, peasants who had produced and sold a surplus on the market just a few years previously were able to do so again, their grain was no longer violently requisitioned as during War Communism, but they did have to pay the Prodnalog (a tax in the form of some of their food output – but a quantity known in advance, and far less than the requisitioned quantitites).

By contrast, the market transition at the end of the Soviet period occurred after decades of total state control. Ordinary people had essentially no memory or experience of a market economy, and were used to what Moshe Lewin called "a low-intensity, low-output" economy. The lack of knowledge was easily exploited during the privatisation phase (e.g. during voucher privatisations, when relatively uninformed and cash-poor citizens were easily persuaded to sell the vouchers which corresponded to shares in former state enterprises), which, alongside the preexisting networks of corruption and mafia-style relations in the Brezhnev-era Soviet economy, facilitated the growth of the oligarchs. Soviet Russia's economy was obviously hindered by productivity and efficiency issues, but these were not always such a huge problem in a Soviet system where 'shadow economy' networks between economic officials and economic relationships between politically friendly Soviet bloc states helped to enable a dysfunctional but basically operative economic system. However, these problems obviously rushed to the fore when Russia attempted to join and compete in the global economy.

TL;DR the NEP was controlled and within living memory of market economic relations, the post-Soviet transition was rapid privatisation of a previously long-term entirely statist economy