Why the taboo against naming discoveries after yourself?

Traditionally naming after scientists has been considered an honour bestowed upon somebody by their colleagues. This is why there are plenty of such names in latin animal or plant names. one very good example is Strigiphilus garylarsoni, a chewing louse, named after Gary Larson, author of the Far Side.

Naming can also be subject to strict laws. In some countries, it is, for example, not possible to name official places after persons until after they have passed away. This is to prevent people to inflate their own reputation while alive (it is quite easy to see where such behaviour is going overboard). But, this is a digression.

The main point is that etiquette indicates that one does not name things after one-self, one can hope that the work is so appreciated by others that a naming occurs. It is probably also a good thing that everything we discover is not named after a person. I drove my Smith at 120 kiloJones per Dickens.

In general, the name of a thing should indicate an aspect of it which is very important to the person assigning the name. A person who creates something and names it after themselves implies that they think the most important thing about it is that they created it. That would in turn suggest to anyone who isn't interested in the person who created it would likely not be interested in the thing thus named.

Thus, naming something after oneself is not necessarily a sign of ego, but rather the opposite. If Alex Johnson (made-up name) publishes a paper entitled "Alex Johnson's Laws of Quarkions", the title would suggest that the paper was primarily relevant in relation to Alex Johnson's other work and would have little relevance outside that. If instead the paper had been simply published "Laws of Quarkions", that would have a stronger implication that the author believed laws described therein to be universal, and thus relevant everywhere.

It is only after the importance of something becomes self-evident that the attaching of the creator's name to it really serves to elevate the status of the creator. Until such time, the attachment of the creator's name will tend to deprecate the importance of the thing thus named.

One reason is that academia tends to frown on self promotion of all sorts (not just naming things after yourself). This is certainly not a universal rule, and some areas are more tolerant of self promotion than others, but it's a good first approximation.

Another reason is avoiding conflicts of interest. A meaningful, descriptive name is better than naming something after its discoverer (imagine if black holes were called something like "Smith objects"). Furthermore, several people are often involved in any given discovery, either as coauthors or as authors of related papers, and it can be tricky to decide who really deserves the most credit. If you let people name things after themselves, they will naturally have a bias to choose that name instead of a more meaningful or appropriate name. Ruling this out of course doesn't eliminate all bias, but it's a start.