What determines the addition of the suffixes "tion" and "sion"?

The distribution of -sion and -tion in English is primarily historical, not rule-based: words that were spelled with sio(n) in Latin continue to be spelled with s, while words that were spelled with tio(n) in Latin are spelled with t (in the past, the letter c was a common alternative to t in words like this).

In Latin, -sio, -sion- is the form that the suffix -tio, -tion- regularly took when it combined with a preceding “dental” consonant (Proto-Indo-European *t, *d or *dh, which generally correspond to Latin t and d). In a few contexts, Proto-Indo-European *dh turned into Latin b, as in iubeo, which has a related noun iussio. But the b in absorbeo does not come from *dh. Since the stem of absorbeo never ended in a dental consonant, it isn't expected to take the -sio variant of the suffix -tio.

The -sio variant was expanded to some words that didn’t originally have a dental consonant, but that didn’t happen in the case of absorptio.

In English, -tion is a more common termination than -sion, so there is not much pressure within English to replace -tion with -sion: in fact, -tion spellings have expanded compared to Latin in the case of certain words that originally had -xio(n) such as reflexion and connexion. In terms of phonetics, there are many other words with /pʃ/; e.g. reception, interception, inscription, corruption; the same can’t be said for /bʒ/.