From the Comair website:
We operate 26 Boeing aircraft, of which 18 we own;
7 x B737-300s
7 x B737-400s
4 x B737-800s
8 aircraft are leased from foreign-owned
leasing companies;
3 x B737-400s
5 x B737-800s
INVESTING in the future, we are in the process of upgrading our entire fleet to Boeing 737-800s. The new aircraft offer a higher seating capacity, lower operating cost and extended potential daily utilisation.
It's better to operate one machine weight limited for one route than to keep a type specifically for that route.
As to the link to CPT, there are so many connections as not to matter and Comair know that there will be many folks arriving in JNB who only want to go to St.Helena (STH??) for a once in a life time trip.