Some portion of Spotify's success, I'd argue, is that they were the first successful entrant in the streaming space. The product isn't particularly remarkable compared to other options and in fact has some notable downsides (such as its ever-changing-for-unclear-reasons UI) but it's what people know and thus what people use.

It having a free ad-supported plan is also a massive factor, but I suppose that would fall under "lowering the bar".

It is not that Spotify is exceptionally good, it is just the others are comically bad.