I would consider "conservative" as a descriptive term rather than
explanatory, so I would say something is conservative because Paleozic and
modern species look very similar.
To some degree, the conclusion of little pressure to change is based on the
long period of time during which they have not changed. Actually, this
suggests some pressure to not change (stabilizing selection). There is
additional evidence for little pressure to change for some groups. In some
cases, it is known that the environment favored by a conservative group is
one that is relatively inhospitable for most organisms, so that once they
evolved a way to exploit the niche they faced little competition (nuculid
clams in very soupy mud). Another possibility is that the organism has an
exceptionally stable niche, and once they have evolved to fit that niche
they are able to outcompete any newcomers (sharks in the open ocean).