In recent years cattle breeders have increasingly used crossbreeding, in part that their steers should acquire certain characteristics and partly because crossbreeding is said to provide hybrid vigor.
(A) in part that their steers should acquire certain characteristics
(B) in part for the acquisition of certain characteristics in their steers
(C) partly because of their steers acquiring certain characteristics
(D) partly because certain characteristics should be acquired by their steers
(E) partly to acquire certain characteristics in their steers

(E) is the correct ans given. Isn't (D) much more parallel with "partly because". Further, the original has "should", dosen't changing that change the meaning, making it a classic case of known/unknown.

It is actually OK for these not to match. "Partly because" is used correctly to indicate the reason for doing something, while "partly to (infinitive)" is used correcly to indicate the goal of doing something. Here we are learning two things about the use of crossbreeding: the goal of doing so (to acquire certain characteristics) and the reason for doing so (because it provides vigor). We have to mis-match to convey the correct meaning.

Also note that "partly because certain characteristics should be acquired..." seems to indicate that cattle breeders are being compelled to use crossbreeding by somebody else who has an opinion about the characteristics of steers. The intended meaning of the sentence is that the breeders are using crossbreeding to accomplish some goal of their own. The use of "should" here is actually a classic example of a choice A that is less logical than some other choice. In such a case, it is OK to go with a choice that deviates from the meaning of the original.