Purge was nothing to do with doctrines. Soviet military theorists were good indeed, but this advanced theories were never implemented in military in large scale.
Someone's misinformed you.
The Red Army field manuals contained mobile warfare doctrine prior to the Purge. This is simply fact, and amply documented. It is there in black and white in both the 1929 Red Army Field Regulations and again, in a completely developed form, in the 1936 Provisional Field Regulations. Guderian refers to it in
Achtung! Panzer in detail, as do a long list of military studies authors. The doctrine was tested extensively in trials conducted at Kazan and Lipetsk beginning in 1926. Incidentally, the Germans also studied and developed much of their own mobile warfare doctrine here - under the limitations of Versailles, they were unable to do so effectively on their own, and the Soviets benefitted from German technical knowledge. Ultimately, however, the Soviets turned to Anglo-American designs and adopted the excellent American Christie chassis for their tanks. (The Germans, for their part, did not suffer when the agreement was terminated - they no longer needed access to Soviet test grounds once Hitler came to power).
"Deep battle", as the Soviets termed the doctrine, was the work of military theorists in the Red Army who tended to be educated men who had a long career in the military, or in academic military theory. Most of them had been educated before the Soviet Revolution, which had only occurred in 1917 (just 12 years before the first appearance of the doctrine in field manuals). Many of them had been educated abroad, in Germany, England, the US, or most commonly, France. When the Purge came, Stalin quickly turned on exactly these sorts of individuals as being counter-revolutionaries who secretly harboured capitalist ideals.
Moreover, he directly targetted the deep battle doctrine. Deep battle doctrine was technological, and relied on machinery, not manpower, but the Stalinist conception of communist superiority believed in the power of mass collective action, even in war. The origins of the deep battle doctrine were based on studies of Russian operations during WW1: the hated Czar's war, which brought the communists to power. The doctrine was also, at least partially, foreign in origin: as mentioned, the Germans had played a big role in its development, while other parts of the doctrine were adapted from readings of British theorists. Most importantly, deep battle doctrine was, as most big ideas are, associated with one man in particular: Mikhail Tukhachevsky.
Tukhachevsky and Stalin already had a personal animosity stemming from a disagreement during the 1920 Polish-Soviet War (Stalin blamed Tukhachevsky personally for the humiliating Soviet defeat), and further, Tukhachevsky was born into an aristocratic family (related to the Tolstoys, apparently). Complicating matters, Tukhachevsky had visited colleagues in Britain and Germany in 1936. He was arrested and executed, and the deep battle doctrine was deleted from future editions of the Field Regulations, on the grounds that Tukhachevsky was a Nazi spy who had foisted an unworkable doctrine on the Red Army, so as to weaken the defences of the USSR, on behalf of his supposed Nazi masters.
And that's the story behind deep battle, and its fall from favour during the Purge.
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