It's a 30 year old format, and large amounts of research and innovation in lossy audio compression have occurred since then. Opus can achieve better quality in like 40% the bitrate. Also, the format is, much like zip, a mess of partially broken implementations in the early days (although now everyone uses LAME so not as big of a deal). Its container/stream format is very messy too. Also no native tag format so it needs ID3 tags which don't enforce any standardized text encoding.

Not the original poster, but there are newer audio codecs that are more efficient at storing data than mp3, I believe. And there's also lossless standards, compared to mp3's lossy compression.