Most of the van services than run on a schedule don't apply for being on Reserve in EWR (those only run on normal business hours.)
>80% of the open trip available to reserves in EWR are in/out of LGA, and generally involve a show of 4:45am and get done around 11pm - no line holder wants them. If they could adjust those flights to normal daylight hours (even giving the option of commuting directly into and out of LGA) those trips would not be so undesirable and dumped into open time so quickly.
It's getting out of hand. As it stands, there are so many reserve pilots having to transfer between the two airports before 6am and after 11pm that it would be to the company's advantage if they just set up an employee bus to LGA at 4am and a return to EWR at 11:30pm. The bus would be full and with people standing.