Why is exclusive breastfeeding recommended for 6 months only?
I guess this is the review (ISBN: 92 4 156211 0) that they are referring to. 


  We found no objective evidence of a “weanling’s dilemma.” Infants who
  are exclusively breastfed for 6 months experience less morbidity from
  gastrointestinal infection than those who are mixed breastfed as of 3
  or 4 months, and no deficits have been demonstrated in growth among
  infants from either developing or developed countries who are
  exclusively breastfed for 6 months. Moreover, the mothers of such
  infants have more prolonged lactational amenorrhea. Although infants
  should still be managed individually so that insufficient growth or
  other adverse outcomes are not ignored and appropriate interventions
  are provided, the available evidence demonstrates no apparent risks in
  recommending, as public health policy, exclusive breastfeeding for the
  first6 months of life in both developing and developed country
  settings. Large randomized trials are recommended in both types of
  setting to rule out small adverse effects on growth and to confirm the
  reported health benefits of exclusive breastfeeding for 6 months.


However, another report suggests that exclusive breastfeeding for a long time may lead to deficiency of some nutrients which cannot be supplemented via maternal diet.


  The dual dependency on exogenous dietary sources and endogenous stores
  to meet requirements needs to be borne in mind particularly when
  assessing the adequacy of iron and zinc in human milk. Human milk,
  which is a poor source of iron and zinc, cannot be altered by maternal
  supplementation with these two nutrients. It is clear that the
  estimated iron requirements of infants cannot be met by human milk
  alone at any stage of infancy. The iron endowment at birth meets the
  iron needs of the breastfed infant in the first half of infancy,i.e. 0
  to 6 months. If an exogenous source of iron is not provided,
  exclusively breastfed infants are at risk of becoming iron deficient
  during the second half of infancy. Net zinc absorption from human milk
  falls short of zinc needs, which appear to be subsidized by prenatal
  stores.


Both these reviews are from WHO website; you can search for different WHO research materials on the site.

Note: WHO does not say that breastfeeding should not be continued till a later stage. As noted in the review, exclusive breastfeeding for more than six months can cause some nutritional deficiencies. However, reducing exclusive breastfeeding to 3-4 months can reduce the immunity of the infant.