I think I might have figured it out:

1. Machinery decreases the SNLT required to produce commodity x
2. The value of commodity x therefore decreases to a minimum
3. Profit can only be increased by extending the working day and producing more of commodity x
4. The constant capital required to invest in machinery cannot be exploited to make profit, unlike the variable capital invested in labor and finally
5. efficient production machinery is only invested in once labor puts a cap on its own exploitation through organization

Haven't read Capital but from a purely worker perspective: having to shut everything down and start everything up is "unproductive" time in the capitalist mind. Maybe it's different now with more advanced machinery but back when Capital was written it must have taken a significant amount of time.