Well there's a big market for classic rock, people of all ages wants to see rock n roll acts but the reality is that the newest bands are really not that good or not that influential they are classic just in sound but not no new band is a classic... I don't know if I'm explaining myself. So, bands started to cover (no pun intented) that niche in order to put some money in the bank. It like... I want a Ford Mustang, the good ones are '65 to '68 (IMO), but they are rare to find and expensive... Imagine if someone started to making copies or "tributes" of that cars at a competitive price, I'll snatch one in no time!