This one is so simple. McIntosh amps are the best sounding, most reliable amplifiers ever made.
During the time frame that the Dead used them, they were also the most powerful amps available.
Think about this, Did Jerry's rig ever fail onstage due to the failure of his Mac 2300? Nope!
Did Jerry fry a few JBLs? You bet he did.
Did Steve Parrish develop a technique for rapidly changing a Mac 2300 on the fly? Nope, no need for that.
That is why Jerry and the Dead used them. There may be comparable amps available today, but during the Dead's touring era, there was nothing as good as the Mac amps.
For example, the Phase Lineier's (also 300 WPC) blew up on a regular basis. There was no Crest or QSC.