Gostaria de saber por que o Selenium IDE não é oficialmente suportado no Google Chrome?
Isso ocorre porque o Selenium IDE é uma extensão do Firefox . O Chrome não suporta extensões do Firefox.
Selenium IDE is an integrated development environment for Selenium scripts. It is implemented as a Firefox extension.
Fonte IDE de selênio
O que posso usar com o Chrome?
Existem ferramentas semelhantes do Chrome (não usei nenhuma delas pessoalmente):
FRET :
Unfortunately the Selenium IDE does just not exist for Chrome. Luckily there is a very good alternative: We suggest you to try FRET as an alternative to the Selenium IDE. It uses the same Syntax as the Selenium IDE, but is web based and runs the tests on machines that belong to the tool which you cannot see. You will only see the test results. The good thing is, that you will also be able to run your tests on Chrome.
WebDriver is an open source tool for automated testing of webapps across many browsers. It provides capabilities for navigating to web pages, user input, JavaScript execution, and more. ChromeDriver is a standalone server which implements WebDriver's wire protocol for Chromium. ChromeDriver is available for Chrome on Android and Chrome on Desktop (Mac, Linux, Windows and ChromeOS).