Sugar harvests were falling below 6M tons in late 1960s which made improbability of 10M tons in 1970 even less likely. All-time record at that time was about 7M tons. 1970 harvest of 8.5M tons remains unsurpassed but it was achieved at expense of general economic well- being (involving mobilization that exceeded health and illiteracy efforts combined). Acknowledged as failure by gov't and subject of self-criticism by Castro, 1970 'zafra' campaign marked end of revolutionary Cuba's 'heroic' period. Idealism, improvisation, experimentation were replaced by instutionalization, structured decisionmaking, rational economic management.
Sugar harvests from early 1970s to early 1990s were in 7M to 8M ton range (7M in 1992-93). But collapse of Soviet Union & Eastern European socialist bloc meant vastly reduced markets (in which Soviets paid higher than world market price) and financing. SU & EE provided 40% of Cuba's trade & aid and their absence left country with no money to buy machinery parts, fertilizers, herbicides for long-time leading source of foreign exchange (now replaced by tourism). Cubans are again cutting cane largely by hand.
Low point was 1995 when harvest was about 3M tons. More recent years have been hampered by effects of Hurricane Lili & El Nino (torrential rains cause yields to plummet). Competition from Ukranian sugar production has limited expansion of European markets. And world market prices have been low for some time.
Michael Hoover (who may be thought a suspect source on such matters and so listers may want to ignore above comments in favor of tried-and-true explanations holding that problems stem from innate inefficiency of centralized bureaucratic planning and agricultural policies of large state farms *and/or* Castro as caudillo)