It is beginning of a fantastic story.
There's lots of single screen platformers out there, or rather, there were. It's not such a hot genre nowadays as making the screen scroll has become quite effortless. And why would you limit yourself to small, square shaped arenas when you can easily make expansive levels? Because none of those 'scrolling' platformers are as fun and replayable as this game is.
A great artist uses very little to do a lot, while a lousy artist uses a lot to do very little. This has always been my philosophy, be it with games, movies, TV shows, almost every one of my favourite works in each category has serious limitations working against it, and it directly correlates with their greatness. In the case of Bubble Bobble, the designers had to do the best they could with a static screen which is how such depth in gameplay was achieved.
With almost any of these games, your objective is to clear out all the enemies, after which you'll automatically advance to the next screen. So in every case, these games are oriented around their combat and always try to make it interesting in some way. In Snow Bros. you can roll enemies into eachother, in Penguin Brothers you can use stage elements to catapult bombs all over the place and in The Fairyland Story the method is to drop cakes onto enemies. Point is, it's never as simple as shoot and kill. And Bubble Bobble takes its combat mechanics to the next level.
There's just two buttons: Jump and attack. The jump goes in a fixed arc and limits your sideways mobility somewhat. The attack is where things start to pop off. You blow bubbles with the primary purpose of encasing enemies that can later be killed by bursting them. So there's already a depth to how enemies are defeated: you can either play it safely and pop them one by one or group them up, letting them 'stick' together you you can burst heaps of them at once. The last way of killing enemies with your bubbles is by doing the indispensable 'kissing' move. This consists of blowing a bubble when an enemy is almost on top of you, which kills them immediately. You'll find yourself having to rely on this technique a lot the further you get in the game, and it can make for some incredible lucky saves.
A good single screener doesn't just need interesting combat though. It's gotta somehow elevate the mobility of its characters in a way that skilled players can exploit. That's where the secondary use of the bubbles comes in - they're a springboard! And this is gonna be your lifesaver in later stages where the geometry of the levels becomes untraversable.
So the mechanics of the character's movement already have several layers that players can learn and adapt to, but that's only one part of what makes the game so replayable. The stage designs themselves are very varied it'll take a a few attempts until you find the right approach for each. You have to learn the air currents through which bubbles flow, where the items spawn and what kinds of special 'element' bubbles appear. Another crucial aspect is to learn how to manipulate the enemies to make them easier to deal with.
We haven't even gotten to the items yet, which appear to come randomized and therefore change on every playthrough. Of course there's a secret to getting the items you want to appear, which again alters the way that you play once you learn it. And the items that you get will in turn change how you're able to proceed in each stage.
What I'm trying to get at here is that all this design work, which I could keep on describing for a while, is far more intricate than your average scrolling platformer, let alone metroidvania. It's not that single screen platformers aren't being made anymore because they're not as good, it's because it's much more difficult to make a small, simple game that's fun. Much easier to just make a game huge and give the player a ton of stuff to do, without having to think about what works and what doesn't. By doing that however, it's likely that the replay value will suffer as is the case with all metroidvania or 'open world' games.
For being the king of minimalist design, Bubble Bobble is my favourite game ever, and I'll still be playing this tiny game when I'm old!
A quick tangent on why I don't like the sequels as much as the original. Bubble Symphony is filled with little things that irk me. Having to collect the notes and the ROD letters is a bother and I wish I didn't have to aimlessly walk around stages or wait around for a minute. Not a fan of the bosses either, as their difficulty is determined by which element potion you happen to get.
Bubble Memories has much more agreeable bosses, but it's got the same 'having to collect shit for the good ending' problem as Symphony. It just feels awful to accidentally miss something and have your whole playthrough go to waste. Also, it's the ugliest of the trio by far! Those backgrounds are disconcerting to say the least. If anyone knows of a hack that replaces them with a black background, please contact me about it!
Lost Cave is an awesome hack, don't get me wrong. But some of these levels have been dug up from the pits of Hell.