I think the time when Srila Prabhupada was preaching, there was counter culture and special attraction towards east which also helped in spreading.
Too much tamas or too much opulence generally makes it difficult for people to accept bhakti.
Queen Kunti says:
SB 1.8.26: Those increasingly under the influence of ambitions for a
good birth, opulences, fame and beauty, will never ever deserve Your
name on their lips, the name of You who can easily be approached by
And also, ISKCON was down for some time after Srila Prabhupada left the world, leadership was in wrong hands for some time, there were some scandals and earned bad name there. The senior most disciple was around 35 year old I think. Such a cultural gap and age gap b/w founder and disciple we see hardly anywhere.
ISKCON is recovering from that phase and hope it will definitely spread. As far as I know, now it is better than what it was before.