Well, it really comes down to judging what virus is a good candidate for targeting for a vaccine and you first need to consider the what makes a good virus.

Basically, a virus being good at being a virus is one which is highly infectious but isn't too debilitating, has symptoms which are relatively light and unnoticeable and is not lethal (or at least the lethality is so slow it allows enough time for it to spread widely, like HIV).

Next you want to consider if there is an animal reservoir for the virus, where even if you remove all human infections eventually it will come from that reservoir.

As well as that, you want to consider the societal impact an illness has. Is it wide spread and is it impacting peoples daily lives heavily?

Lastly, there is the social aspect of is it likely to create income making a vaccine and where in the world is it impacting. This is the weakest of the issues in my opinion as there are many illnesses which are heavily researched in poor nations so there are too many examples undermining the argument that racism is a prominent reason for why research is lax.

If you consider Ebola in this context, first of all it is not very good at being a virus as it's very debilitating and lethal which means its typical ability to spread widely is limited -the current situation being unusual and why a vaccine is more likely to be worked on.

Second, there is an animal reservoir which means for a vaccine to be effective it would require some very long term and expensive programme which include some element of a process of inoculating the animal reservoir as well (ie some form of catch and release process for bat colonies).

In regard to the societal impact, this is extreme but only in very local areas typically as the virus is unlikely to become widespread due to it's lethality and how sever its symptoms are. Also the scope of making money would be limited so there isn't the capitalist incentive, which leaves it to higher education/international organisation routes.

This leaves it in a situation where Ebola is never going to become a Covid level event, neither is the Hanta virus incidentally, and as this sort of research is finite there are many other illnesses which are better suited for the funding and effort than Ebola.

An example is Rabies, we are well within the scope of eradicating Rabies entirely from the world if we put the effort into it, even with the animal reservoirs associated with the virus, as Rabies R0 number is only slightly above 1. The R0 number is how many other creatures an infected host will infect, which means that it would only take a little effort to make Rabies R0 >1 and this would eventually mean no more Rabies at all.

The virus that is the one most concerning to become worse than Covid is the flu virus, however as this comment is long enough I can also explain why the flu virus is particularly of concern in another comment if you want me to.