Why is a racemic mixture formed in the Diels-Alder cycloaddition?
The concerted Diels-Alder reaction has in principle eight different possible transition states. Depending on the substrates some of them will lead to the same products.

Below you can see an overview of the (non-optimised) transition state arrangements for the reaction of isoprene and propene. 



From these images you should be able to determine that the following behave like enantiomers:


(a) and (g)
(b) and (h)
(c) and (e)
(d) and (f)


When you think a little bit longer about it, you will also see that (a) and (h) will form the same product. This is also true for (b) and (g), (c) and (f), as well as (d) and (e).
In total, you will therefore obtain four products two different regio-isomers and their corresponding enantiomer each.