Also depends on the nature of the chemical and the desired results, and how anxious the developers want to bring it to market. For example, certain compounds are easy to inject, but developing an oral formulation (which are generally more desirable) would take longer. But if the drug is the first drug in a new class, the first to market status could trump oral availability.

Also, sometimes drugs are for specific applications - there is no point in testing a topical agent orally. Or the rectal absorption of a drug intended for join injections.

But sometimes it happens that a smart person thinks "hey, this drug I've been injecting could work on hemorrhoids." So then that might be studied.