HJP is much bigger town than SEE and there are a lot of people traveling between patna and hajipur. Extension would have increased the passengers by substantial amount. Only a small part of main line will be used so it won't be much of a problem. There are a lot of trains from SEE to HJP but no one would like to change trains for 20 km journey. I guess lack of PFs at HJP is the only reason for its non extension.