Well, the way I take it is that just as humans range from those not suffering and enjoying life to ones who suffer a lot every day, and also animals also - then when you think of the range of all beings that might exist anywhere in the universe then the range of conditions must vary even more. For instance there are some things we simply can't experience or our bodies would die.
So - if your life is too blissful, it might not be that easy to relate to the Buddha's teachings, because - like what's the problem with this life? It's hard to relate to its ending - that none of our pleasures are permanent and lasting. You might come to see that but still, on a rather intellectual plane, you know it's true, but it doesn't have the same urgency. Which you might also get from seeing others suffering - but again - might be not quite the same as experiencing it yourself to directly motivate you to take a wider perspective on things.
But if you are suffering too much, then it's hard just to think about anything at all except your suffering. So again you are somewhat blocked from listening to the dharma. And it might be hard to see how the teachings could help, but also, if in extreme pain not easy to keep a coherent chain of thought either..
In between though, then you experience suffering, but also can reflect on it, and can listen to the teachings and relate to them.
And in the stories then the Buddhas do teach to beings in all conditions, heaven realms, hell realms, animals also, each in the way they can relate to. But humans in our present form are perhaps the easiest to teach or amongst the easiest because we both experience suffering, but not too much, and are able to reflect and listen.
With a lot of variation also between people of course, and it's not just your physiology and temperament - it's the connections you have.
So, can be a human but the teachings that come to you from your family or society, the way "things are always done" that you accept as standard - depending where you were born, that might help to open you to the teachings - not just of the Buddha of course, but generally teachings on love, compassion, wisdom and so forth - or they might be somewhat of a block e.g. so that they encourage greed or hatred or violence and murder so you come to think of them as things to try to nurture and increase in your character
If you are brought up from when you are tiny to respect and admire violence and greed, and to treat people with contempt who are not like that, then it's gong to be hard to relate to the dharma just because of your upbringing.
If that happens, you are in the situation where you have the physical capability to listen to the teachings, and the mental capability for coherent thought so you can reflect on them - and are not in extreme pain or blissed out - and could understand the message - but because of your upbringing is almost impossible to listen to it because you just dismiss it out of hand as not aligning with what you were brought up to understand and respect.
So - it depends a lot more on us than on the Buddhas. If they could teach us they would. But we put up numerous blocks, because, in our own way, we are as powerful as the Buddha - but our power is put mainly into creating delusions and obstacles to the clear message of love and compassion, truth and wisdom.
So - then the Buddha's don't give up - but the way they act is subtle, just hints and suggestions and often doing strange things that don't make much sense to the rest of us - that's the tradition of the "divine madman" you have in so many religious traditions, people who seem mad but actually they are teaching, may be recognized within their tradition also sometimes as great religious teachers but doing things that just looks crazy. Because we can't relate to the direct simple teachings, and in some way this is their way of helping others - though it's hard for us to understand how and why in our limited blocked off view of reality..
And they can't make anyone think one thing or another, you can't "enlighten" someone - it has to come from them, that they want it and search for it.