The important historical reason is that Trotskyism is opposed to the worker-peasant alliance. That's an untenable position now given the events that happened after Trotsky (in Vietnam, China, etc)
The leftist squabble reason is that Stalinists and people aligned with them are mad at Trotskyists opposing them.
I appreciate your explanation. which had me reading one of Trosky's writings
that goes over his thoughts of the Chinese revolution and of which i find somewhat valid but the snipping in this spoiler also preemptively defending his position.
We must turn to all the members of the official Communist Party with words of explanation and challenge. It is quite probable that the rank-and-file Communists who have been led astray by the Stalinist faction will not understand us at once. The bureaucrats will set up a howl about our “underestimation” of the peasantry, perhaps even about our “hostility” to the peasantry. (Chernov always accused Lenin of being hostile to the peasantry.) Naturally such howling will not confuse the Bolshevik-Leninists. When prior to April 1927 we warned against the inevitable coup d’état of Chiang Kai-shek, the Stalinists accused us of hostility to the Chinese national revolution. Events have demonstrated who was right. Events will provide a confirmation this time as well.
It's Bolshevism that tries to blame every issue with the USSR on Stalin while ignoring that many of the structures that eventually led to Stalinism were present earlier on, when Trotsky and Lenin were at the helm. As a consequence, non-Leninist socialists, such as anarchists and autonomists, tend to dislike Trotsky and as a consequence Trotskyists.
Although I still have material to read, my current thought is the following: Trotsky was closer to the Mensheviks than to the Bolshevik faction; or if you prefer, his approach was situated somewhere between both currents. If he joined the Bolshevik faction, it was out of pure pragmatism.
If you read Lenin in 'What Is to Be Done?' or 'The State and Revolution,' you will see how he criticizes the Russian social democrats of that time, from the opportunist branch (referring among others to the Mensheviks and similar), for wanting to collaborate with the government and divert the proletariat from the path of revolution. Stalin does nothing more than follow the path started by Lenin.
Hence, the 'orthodox' thought of Marxism sees Trotsky as a revisionist/reformist.