One reason might be that Steppenwolf have successfully managed to manhandle their reputation through awful management decisions and zillion unoriginal line-ups pretending to be the band.
This whole RRHoF what-aboutism ("if this band is in, then why isn't my band ...") leads nowhere. I'm fine with most bands and artists inducted there (for me "RnRHoF" simply stands for "all popular music since the 50ies" and includes pop, blues, singer/songwriter, reggae, soul , funk, rap/hip-hop, country and what have you besides "real rock" and its sub-genres such as prog or metal), in one way or another most of them deserve their place. I scratch my head about some omissions but that doesn't take anything away from those artists already inducted. Yes, Steppenwolf should be featured too because a term from one of their lyrics, namely "heavy metal thunder" christened a whole genre and the movie Easy Rider and its soundtrack is a late 60ies cultural icon. They were also a prototype heavy rock band with their dominant guitar & organ sound and my countryman John Kay's tough vocal delivery.