The point is that each node needs to have a route to all the subnets, either a default route (hosts) or a specific route to each subnet (routers).
When a node is directly attached to a subnet, a specific route is set up automatically. Otherwise it needs to be set manually by the admin, or learned via a routing protocol from peer gateways. (Hosts often have a default gateway set by DHCP.)
Case 1 is trivial - likely the hosts have their default gateway set to their router's southward interface. That single router is directly attached to all subnets, so all necessary routing table entries are added automatically.
In case 2, hosts are likely using their router interface as default gateway as well. But unless you've explicitly set up the routers, they don't know the subnet behind the other router. Each router has got an automatic entry for the directly attached subnets - the client subnet below and the link to the other router. However, there's no route to the subnet behind the neighbor router, so that is unreachable.
You could set up each router on the other one as default gateway, but that would prevent any more routers to be added.
Much better, you either set up a static route to the remote network or you set up a routing protocol like OSPF between the routers.