Especially when they both derive from Endodontist and Dentist respectively, so context doesn't vary much.
I found many answers on the web explaining that -ic and -ical suffixes don't follow any rule as such and that -ology words tend to favour -ical and never use -ic, but the -al suffix is largely ignored in most discussions.
The adjective-forming suffix -al is originally from Latin (in the form -alis). The adjective-forming suffix -ic existed originally in both Latin (in the form -icus) and in Greek (in the form -ικός), but Greek -ικός was used more freely to form new adjectives than Latin -icus. So a large proportion of -ic words are from Greek, while a smaller proportion of -al words are from Greek. (The combined suffix -ical developed later as an extension of -ic, so like -ic, -ical also tends to occur in words with a Greek origin.)
Dental is built on the Latin word for tooth, dens (with the stem dent-), while endodontic is built on the Greek word for tooth, ὀδούς (with the stem odont-).
Dental does not derive from Dentist but they both derive from the root 'dens'.
There is no specific reason for the suffixes being different here, because languages are too complex and each suffix has multiple meanings.
