So I'm coming to this question rather late, but there are a few reasons for this. Firstly I will say that I am not very familiar with Byzantine history post Justinian, but if I'm not mistaken I believe they resorted to blinding (and maybe castrations) to deal with deposed emperors. That being said I can offer an explanation as to why assassinations "fell out of favor", so to speak, in late antiquity.

​

So to start, the Praetorian Guard was responsible for a lot of those assassinations. They assassinated at least 8 emperors, and possibly as many as 14 (if my numbers are correct.) In total 38 Roman Emperors, including those whose deaths were questionable, were murdered up to 476. I know that goes beyond the date of your question but I figured I would include all of the ones murdered before the "fall" of the western empire. But anyway, one of the major reasons that assassination became less common after 284 (especially if we're setting aside those murdered by their own troops) because Constantine disbanded the praetorian guard.

​

As I've already established the praetorian guard was in the business of assassinating emperors that they didn't like. The praetorian guard also had easy access to the emperor since they served as his body guards. The problem, however, is that the praetorians did not answer to the emperor directly, rather they answered to the praetorian prefects, who in turn answered to the emperor. This often allowed the prefects to amass political power and influence. A good example of this would be Tiberius' praetorian prefect, Sejanus. When Tiberius withdrew from public life Sejanus was effectively the most powerful man in Rome and even murdered Tiberius' heir. Eventually Tiberius had Sejanus executed and replaced him with Macro, who might have in turn assassinated Tiberius.

​

Now you might be wondering why the praetorian guard wasn't disbanded sooner considering how often they interfered with imperial politics, and its probably because they simply held too much power. In fact several emperors resorted to giving the guard, and the military as a whole, "gifts" to ensure their loyalty, so effectively they bribed them. That being said Flavius Valerius Severus did try to get rid of the guard but they murdered him for his trouble.

​

Constantine was likely only successful in his attempt to dissolve the praetorian guard because his victory at Milvian Bridge weakened their ranks, and they no longer had the man power to resist him. In this wake of his decision to disband the guard Constantine created a new imperial bodyguard, the scholae palatinae. We aren't quite sure how many men were in the scholae palatinae under Constantine, but it was 500 in later years so it was probably a similar number during his reign. The praetorian guard on the other hand fluctuated in size. anywhere from 4,000-15,000 men. This effectively made the praetorians their own army in late antiquity, which also gave them greater security and influence. The scholae, however, could not rely on their military strength if they decided to assassinate a emperor, and very easily could have been wiped out by any loyal contingents in the army (assuming the assassinated emperor wasn't despised by his soldiers as well).

​

The other thing that made the scholae a more reliable bodyguard is that they answered directly to the emperor himself. Now to be clear they did have officers, but no one man commanded the entirety of the scholae. This largely meant that they were more loyal to the emperor since he commanded them directly, and there was no military commander who could use his position to gain political power and influence for himself.

​

Sources:

Roth, Jonathon P. Roman Warfare. New York: Cambridge University Press, 2009.

Southern, Patricia, and Karen Ramsey Dixon. The Late Roman Army. New Haven: Yale University Press, 1996.

Southern, Patricia. The Roman Army: A History, 753 BC-476 AD. Amberly Publishing: Gloucestershire, 2014.