As mentioned above, 7.3 is already out of active support, and in December will be end of life.
Given we have multiple dependencies which needed to be bumped up to support PHP8 requiring 7.4 as a minimum, and several of which are backwards compatibility breaking changes (so had to be released only in a major release), there was no alternative but to drop 7.3 support.
Unless of course you want us to not release support for 8.0 for another ~12 months (when our next major release will be) which is quite a significant issue for the many hosting providers and Mautic users who, for security reasons, are required to only use versions under active support.
I appreciate it means you have to update your servers which may come at some inconvenience, however do you really want to be running an unsupported version of PHP which is not going to get any bug fixes or features, and is going to be completely unsupported in four months time anyway?
Time to plan those updates a bit more in advance before the end of active support perhaps