If chown can change groups, why was chgrp created?

chown initially couldn't set the group. Later, some implementations added it as chown user.group, some as chown user:group until it was eventually standardised (emphasis mine):

The 4.3 BSD method of specifying both owner and group was included in this volume of POSIX.1-2008 because:

There are cases where the desired end condition could not be achieved using the chgrp and chown (that only changed the user ID) utilities. (If the current owner is not a member of the desired group and the desired owner is not a member of the current group, the chown() function could fail unless both owner and group are changed at the same time.)

Even if they could be changed independently, in cases where both are being changed, there is a 100% performance penalty caused by being forced to invoke both utilities.

Even now, chown :group to only change the group is not portable or standard.

chown user: (to assign the primary group of the user in the user database) is not standard either.