ELP sold 40-50M albums worldwide, how is this "weak sales"? They sold similar number of albums to likes of Styx,REO Speedwagon, more than likes of Kansas (30M), pretty good for a band that purposely not aiming for singles...
I also do feel ELP is not that strong candidate, but could you please do some basic verification before you make your idiot takes. Most people here can't stand your idiot takes.
As a technical guitar workout, glam metal sits on a completely different level than grunge.
The genre was built around virtuosity — fast alternate‑picking, tapping, sweep‑picking, wide‑interval legato, and harmonically complex solos. Players like Vito Bratta (White Lion) weren't just good - listen to his solo on "Wait". His phrasing, tapping fluency, and harmonic sophistication put him leagues above any guitarist associated with grunge.
Even the best grunge lead player — someone like Mike McCready — is, in terms of pure technical facility, middle‑of‑the‑pack compared to glam‑metal players. He's expressive, melodic, and tasteful, but he's not operating at the same mechanical difficulty level as Bratta, George Lynch, Warren DeMartini, Nuno Bettencourt, or Jake E. Lee.
Odd time signatures have nothing on pure speed. Speed kills. Glam metal is generally played at nearly double the BPM of grunge. Where grunge often sits in the 90–115 BPM range, glam metal frequently pushes 140–170+ BPM, with dense 16th‑note rhythms and extended solos. That alone raises the technical bar.
You really can't justify Grand Funk Railroad as an inductee. Never had critical praise (Closer to Home is their best album). They weren't a strong legacy tour act past 1975. They had a short peak (69-75) and have a weak signature song in "We're an American Band" as their cover of "Some Kind of Wonderful" has more streams. They sold 25m so nothing amazing.
ELP does not rate highly for me either. Weak sales. Decent touring when an active band, but reunions / legacy tours did nothing. Better critical praise (Trilogy). Terrible streaming numbers. Weak hits.
Realistically, HOF has many bigger snubs than either band and nowadays these bands have no name value.