There's maybe some very complex grammatical reason, but it may be easier to study them as collocations. Sometimes the certain preposition that is used with a certain verb or sentence structure is almost arbitrary. A person is ‘investigated FOR something/doing something,’ whereas a person is ‘accused/suspected OF something/doing something.’ It's the same for English speakers when we try and learn Slavic languages–some verbs almost arbitrarily use a certain case or a preposition. When ‘suspect’ is in noun form, we just arbitrarily use ‘IN.’ ‘He is the suspect IN the commission of a crime/in the robbery/in the arson case.’ When it's the verb form of ‘suspect,’ OF is used: the person is ‘suspected OF committing a crime/of wrongdoing/ of stealing a car, etc.’ Practically, it's just different ways to formulate the same kind of sentence. Out of curiosity, it looks like in Russian and Ukrainian, when talking about a person being a suspect/suspected of smth, the thing they're suspected of stays in the same case and with the same preposition. But, maybe it could be expressed analogously to the noun/verb formulations in English: Suspect as a noun: Он является подозреваемым в совершении преступления / Він є підозрюваним у скоєнні злочину. Suspect as a verb: Его подозревают в совершении преступления. / Його підозрюють у скоєнні злочину.

The prepositions "of" and "in" are used differently in the phrases "suspected of committing" and "suspect in" because they serve distinct grammatical roles in each context. 1. Suspected of committing "Of" is used here to link a person to an action or crime they are thought to have committed. In this structure, the verb "suspected" is followed by the preposition "of" to indicate what the suspicion is about. Example: He is suspected of committing the crime. The focus is on the specific act or crime. 2. Suspect in "In" is used to indicate involvement in a broader context, such as a case, investigation, or situation. Here, "suspect" is a noun, and the preposition "in" links the person to a particular case or event. Example: He is a suspect in the robbery case. The focus is on the person’s involvement in the overall case. In brief ,"Suspected of" is used with verbs (actions), referring to what someone is suspected of doing. "Suspect in" is used with nouns (cases or events), referring to the involvement in a particular situation.

You should get in the habit of including more context with this type of question. We definitely need more context for the second example because "suspect" could be a verb, a noun, or an adjective. In the first one, "suspected" is the past participle of the verb "suspect." It could be serving as the main verb of a sentence in the Passive voice, or it could be serving as an adjective. In either case, the prepositional phrase "of committing [+ the name of the crime]" is serving as an adverbial phrase that's defining "suspected."