The free trials are for new subscribers only. This is mentioned in a footnote in the Apple Music page here.
As you had a subscription in the past you are no longer a new subscriber, even if your subscription was for free. This is no doubt to prevent users from continually signing up for a succession of free trials.
You will note also the similar note for trials of Apple One, the service bundling together half a dozen of Apple's subscriptions: The Apple One free trial includes only services that you are not currently using through a free trial or a subscription. This means that you can't suddenly get a free period of a service to which you are already a paying subscriber or extend a free trial period already underway.