From that article, it looks like if you were to inject a harmless, dead virus into your body, your body would largely ignore the harmless thing. An inflammatory adjuvant is added to the injection to purposely cause inflammation around the injection site, so the body associates the harmless virus with harm, thus developing immune memory.

So injecting most vaccines intravenously would spread the harmful adjuvant widely throughout your body, which is a risk you wouldn't want to take unnecessarily. Injecting into muscle is known to work well and be relatively safe.