The reason is the ER-2, which was specifically remanufactured by Lockheed for high altitude weather research (whereas the WB-57s are "general purpose scientific platforms"), has a 12 hour endurance while the WB-57 can only stay up for 6.5. Because of this, the ER-2 and DC-8 are paired together when doing long-duration weather research flights since they have compatible endurance meaning they only need those two aircraft instead of 1 DC-8 and 2 WB-57s.
Edit: Also, looking at the OLYMPEX website -
http://olympex.atmos.washington.edu/ , the ER-2 is actually a "partner program" that is flying a different mission that is in support of OLYMPEX's flight operations -
Quote:
RADEX
An important partner within NASA is the RADEX (Radar Definition Experiment), which will make radar and passive microwave measurements with the ER-2 aircraft in coordination with the OLYMPEX DC-8 and Citation aircraft (see Section7below). The goal of RADEX is to obtain information that will advance the development of radars to monitor clouds from space, including where cloud processes are active in thin clouds and a wide range of ambient temperatures. These goals lead to the desire for the same type of measurements that are sought in OLYMPEX, so OLYMPEX and RADEX will work very well in a joint experiment.