Because he's still not exempt from the "danger to public health" clause. If he has a strain of drug resistant TB for example, it would be a problem.
5.19. Medical requirements
Members of the family class are medically inadmissible if they or their family members are likely to
be a danger to public health or to public safety or if their admission might reasonably be expected
to cause excessive demands on health or social services [A38(1)]. See Exceptions to medical
inadmissibility, Section 5.20 below.
...
5.20. Exceptions to medical inadmissibility
A38(2)(a) states that spouses, common-law partners and dependent children who are members of
the family class are not inadmissible even if they have a medical condition that will result in
excessive demand to health or social services.
Note that this goes for spouses and children
being sponsored, not when a family is applying together for example under skilled worker class.