PEs routers connect to the P routers with Default-route , PEs can ping each other , but Customers connect to PEs can't access each other.
customer can't ping the other Customer in the same VPN
to establish a correct LSP you have to find in the routing table a matching route for the other peer of the LSP , so in this case you can add static route in PEs for loopback and point to P router, or you can advertise the PE's loopback in the IGP
PE1 access the PE2 loopback through the default route advertised by the P1 router, so there's no exact route in the routing table for PE2's loopback address, so the LSP path between PE1 and PE2 can't be established,you can check that by :
display mpls lsp
you will not find a LSP path to PE2's loopback address