He wasnt forceful because Labour's working class base's sentiments towards Eu was mixed. Therefore he didnt take a definitive stance, in accordance with the sentiment of its base.

Its a statistical reality that Eu immigration caused a lot of job loss in low-education job sectors and this affected a noticeable portion of working class Brits.

Its also a reality that Eu membership helped a lot of better off and educated Brits, which were also found among Labor's voter base.

So the situation was mixed, Corbyn's approach reflected the mixed reality. Like any decent political party anywhere in Europe.

But as can be seen the majority of lower socioeconomic segments wanted Brexit, and they did anything to make it happen.