Looking at some statistics the gap wasn't that small actually.
F.e. from Jan-May 2021 Memb had on average 500-600 viewers, while Nili had 700-1300, in April/May was DMWC but even if we only look at Jan-March Nili had still 700-1000 and this is not cherrypicking since Memb also had months with 300-400 Viewers on average at that time.
One of the big differences between Memb and others is proly that he streams an insane amount of time each month, even for the biggest Memb-Lovers watching 250 hours a month seems impossible, especially if you watch other aoe streamers aswell.
So watching Memb might have felt like smth you could do anytime, while watching someone who's streaming way less feels more "special".
But now Viper is gone, T90 is gone, Hera is gone, Nili is (expect for later Stages of Tournaments) gone aswell.
So People found the crazy Spaniard, enjoyed his streams and decided to stay.
Also he's the biggest Tournament Organizer in AoE2 atm, 3 S-Tier Tournaments announced for this year.
I think that instead of comparing averages (of course, Memb does stream more hours) we should compare peaks, especially when both are casting the same.
I recall that Nili used to have A LOT more viewers