Why is the Jacobsen's catalyst with tert-Butyl groups a better catalyst for epoxidation?
This type of oxygen transfer reaction consists of a two step
catalytic cycle, wherein, first an intermediate (salen)$\ce{Mn^V}$ oxo complex is generated, and then the activated oxygen is added to the olefinic double bond.

Electron withdrawing and electron donating group influence the strength of the $\ce{Mn=O}$ bond, with electron withdrawing substituents leading to a weaker $\ce{Mn=O}$, and electron donating substituents leadings to a stronger $\ce{Mn=O}$ bond.

An argument based on Hammond Postulate can be applied where in the case of electron withdrawing substituents (weak $\ce{Mn=O}$ bond), one expects an early transition state, and in the case of electron donating substituents (stronger $\ce{Mn=O}$) a late transition state is seen. 

Enantioselectivity is governed by interactions of the salen ligand, which in the transition state geometry adapts a folded geometry forming a chiral pocket, and the incoming olefin. 

A late transition state demands a certain proximity between the
olefin and the manganese oxo complex. This leads to a less  spatial separation between substrate and catalyst and consequently allows for better differentiation of stereoisomeric transition structures. 

Thus the attenuation of the reactivity of the oxo-species by electro donating groups, leading to a comparatively late transition state is the reason for higher enantioselectivity.



References

This article looks at the electronic effects of 5,5' substituents using DFT calculations.
Cavallo, Luigi, and Heiko Jacobsen. "Electronic Effects in (salen)Mn-Based Epoxidation Catalysts." - The Journal of Organic Chemistry, 2008, 68(16), pp 6202–6207