I've been told there is an exact Russian-created reason for the limited use of Ukrainian in eastern Ukraine (Russia controlled eastern Ukraine):
Emz' Decree, and
Valuyev's Decree:
The Ems Ukaz..., was a secret decree (ukaz) of Tsar Alexander II of Russia issued in 1876, banning the use of the Ukrainian language in print, with the exception of reprinting of old documents. The ukaz also forbade the import of Ukrainian publications and the staging of plays or lectures in Ukrainian..."
Valuyev's Decree"...Interior Minister Count Pyotr Valuyev issued a decree through an internal document circulated to the censors on 18 July 1863. Valuyev's circular implemented a policy based on his opinion that "the Ukrainian language never existed, does not exist, and shall never exist". It banned publication of secular and religious books (apart from belles-lettres), on the premise that not only is the content of such publications potentially questionable, but their very existence implied the anti-imperial idea that a Ukrainian nation could exist."
As well, Russia has gone out of its way to limit, and band the use of the Ukrainian language (in print and in education) in eastern Ukraine for centuries:
http://www.lucorg.com/UserFiles/File/Prohibitions.pdf
Although Western Ukraine also had bans on their language and education in Ukrainian, they somehow kept the language and culture alive.
The Eastern Ukrainians have been pressured far more extensively to become Russian speakers, for far longer than the Western Ukrainians.