First because enterprises need a company behind offering support contract.
But more important, many enterprises are slow moving and unfortunately Debian lifecycle is not long enough for enterprises because of that..
I have at least one customer that migrated from RHEL 6 into newer versions last year, RHEL 6 was released on 2011, regular support ended on 2021 and extended support goes until next year. So it is currently still supported under their extended support.
The Debian version from 2011 was Debian 6 (Squeeze) that had an end of life in 2015, and long term support ended on 2016, it did not had extended LTS.
Just on the lifecycle of RHEL 6 Debian released versions 6, 7, 8, 9, 10, 11 and 12 and with exception of the last two, 11 and 12, all other versions already reached end of life, additionally version 6 to 9 reached end of LTS, and version 7 reached end of ELTS, older versions did not had ELTS as far i could find.
Is is also worth noting that Debian LTS and ELTS are not handled by Debian security team, so anything after end of life would be a solid no from most enterprises.
Even Ubuntu is behind RedHat in this aspect, the oldest Ubuntu LTS release still supported by next year, when extended support for RHEL 6 will end, is Ubuntu 14.04 from 2014. Both Ubuntu 10.04 from 2010 and Ubuntu 12.04 from 2012, that is the last LTS release on or before 2011 and the following release, already reached end of life and support before RHEL 6 extended support ended.
[0] - https://wiki.debian.org/DebianReleases [1] - https://ubuntu.com/about/release-cycle