>>11961484
>I guess they were just learning how to program for Gameboy.

The standards on Game Boy evolved with time. You need to realize something, SML came out in August 89 in the US. SMB3 hadn't even been released in the US yet, nor were many NES classics.

Having a portable console that was like having a pocket NES was already amazing on its own. That and the GB was cheap and the games were half price compared to NES games.

If you look at the GB libraries Nintendo always seemed to set the standards others will follow. Early on you had games like SML, something like Batman - The Video Game is very inspired by it and that was all that was needed.
With time things evolved as they do on all platforms and by 92-93, after SML2, games were getting more detailed and technically more advanced. Until 95-97 where the platform was getting very few releases before Pokémon made it cool again.
Then for the GBC Nintendo set a new standards: instead of new games specifically made for the platform you get for the most part re-releases and rehashes (but in COLOUR!), shitty ports of console games, and so that's what others did too. The GBA would get a bit of both (some new things, some shitty rehashes and ports) but Nintendo would never put the same level as it did with the early years of the GB to create plenty of new games specifically tailored for the hardware to make it stick. This is what SML represents.