At that time, no one had sound systems suitable for large indoor arenas or outdoor stadiums. The Beatles, who had performed live for years in smaller indoor places, realized how poor they sounded in the large venues. After their 1966 concert at Candlestick Park in San Francisco, they vowed to never play live in such locations again. That, of course, meant they wouldn't play live anywhere again.
Gradually, during the second half of the 60s and on into the 70s there was a lot of work done to make better sound systems, especially for outdoor performances. In particular, the Grateful Dead spent a lot of money to help develop this. Some of the notable people who took part in this were Dan Healy, John Meyer (of Meyer Sound), and Owsley Stanley (both a famous genius & an infamous nutjob). Without their efforts, good sound systems might still be no better than what the Beatles and others had in the mid 1960s.