The only reason would be a low grade hosting service. News and blog software use little In the way of resources, any half decent host will suffice. Forum software such as used here can be resource hungry but only if it gets regular moderate usage (20+ online at once). As an example I run a dedicated server for a fairly large model flying community which has 15000 members and regulary has 150 or more online at any one time. This requires a dedicated server to run correctly but in earlier times when we had only 20 online at a time a half decent hosting package would suffice.
Gerry