I dont get why some apps depend on Google Play functionality. Why can't I download Telegram on /e/OS
At any rate, microG is a reimplementation of Play Services.
You can directly download the Telegram APK from: https://telegram.org/android
(Or using the app store that is part of /e/OS and serves as a front for both the Play Store and F-Droid.)
The real reason to me is that they want to avoid Android forks outside of their control.