Why was Stalin able to come to power in 1929?

There are a number of different points that can be used to answer this question and it should be noted that they all combined together to enable Stalin to take control. In the case of this question, the points cannot be split into long and short term causes to structure the essay but it would be fine in this case to simply split the points into separate paragraphs. Despite the importance of all contributing factors it is important to argue for a more/most important cause. In this case I would argue for the way Stalin dealt with his rivals, in particular Trotsky, enabling him to gradually remove rivals until only Stalin and his supporters remained in the Politburo and any rivals that remained were virtually powerless and without support. Stalin told Trotsky the wrong date for Lenin's funeral ensuring Trotsky would miss it and look worse, he played members of the Politburo against each other making and leaving allies whenever benefitted him more. However, alongside this are Trotsky's own weaknesses which served to make Stalin look better and Trotsky had unfavourable policies opposite to what Stalin was proposing (eg. World Revolution vs Socialism in One Country). Furthermore, Stalin was General Secretary of the party and so he was able to fill the Politburo with supporters loyal to him.