Dear comrade Mirza,
For marxists individuals play a dual role in history. First and foremost, the so called leaders tend to express the relation between social classes in society.
Individuals canīt force their will on society and are not free agents who can do whatever they want, but are limited first and foremost by material development of the society they are in and are of course also limited on the other hand with the relation of social classes.
For example, Lenin and Trotsky in 1909 did not have influence to "spark revolution" just like that. However when there was a possibility they had a totally key role in the process, so without them, in the last analysis, there would be no October revolution, but also they themselves were not sufficient when there was lack of the objective condition. So this is a dialectical relationship between history and individuals.
In the case of Trotsky and Stalin it was the case that objective conditions were on the side of Stalinist faction. That does not mean that Stalinists were in any way better intellectually, they were quite inferior in most cases, but the objective coditions of backwardness and disillusionment of masses had beaten the subjective role of excelent individuals as was Trotsky and his comrades.
I suggest you check out and read following two links:
http://www.marxists.org.uk/archive/trotsky/works/1936-rev/ch05.htmhttp://www.marxist.com/russiabook/part2.html (this part also includes your very question -
Why didn't Trotsky take power?However, I would these parts are part of a whole, so I think you should consider reading these books, which can be found here:
http://www.marxists.org.uk/archive/trotsky/works/1936-rev/index.htmand
http://www.marxist.com/russiabook/comradely,
Luka