Because repeaters utilize two channels at once (input and
output) and extend the operating range of a single user,
their use would limit the number of users able to share
these frequencies at the same time.
...
some commenters are concerned that MURS frequencies will be
congested and that repeater use will only aggravate this
problem. We agree.
Basically, they want to maximize the number of users that are able to reasonably use this band. At the time (1998-2002), two-way radios were much more popular than they are today (cell phones have largely made them obsolete), so I can understand the concern.
For whatever reason, MURS never did became as popular as GMRS or FRS did.
