I think the reason why they got delisted was either due to low trade volumes or due to the dev miner fee where a large percentage of a mined block was sent to the developers wallet.

From what I recall the block reward was 314 MUSIC and later it was reduced to 250 MUSIC where the difference went to the dev wallet.

This created an issue because it pretty much was no different than having a premine and generally coins which have premine are similiar to ICOs and most of those were labelled as securities and since Bittrex is in USA they need to comply with SEC.

Most likely if they never did the forced dev block miner fee, then they would of remained listed.