Because between drow, driders and aranea, there weren't quite enough freakish pseudo-anarchids yet.
Ettercaps have five aberration HD (ouch), decent stats, a very small natural armor boost, a climb speed, a natural bite attack (with pretty nice poison) and two claw attacks (dealing an overwhelming 1d3 damage).
They can also throw webs, which is a reasonable save-or-suck for certain foes. Perhaps more interesting is their ability to create large sheets of webbing to walk on or as traps, but well, it's not like web-creation is exactly hard to get at ECL 5.
In the end, I just don't think a LA is needed, seeing a druid with Aberration Wildshape can turn into one at ECL 6 already (if you feel druid is an unfair comparison, use wildshape ranger instead). Even if that weren't the case, ettercaps would just be underwhelming. -0 LA.
Next are ettins!