Why are the Infinity IRS, Reference 1b, Beta ect speakers passive on the mids and highs?

I think it's a practical matter. How many amplifiers do you want to have in your system? While it makes practical sense to separate the bass frequencies from the rest of the speaker system - especially when the manufacturer has already committed to using separate bass towers, as in the RS-1 and various IRS systems - there's less advantage to be gained by separating the mid from the high frequencies.

Separating the bass and using an active x-over also allows use of a servo system for those frequencies, which Infinity did with great success. I don't think it's feasible to use a servo on higher frequencies, however.

Why are the Infinity IRS, Reference 1b, Beta ect speakers passive on the mids and highs?

I think they found as I did. (with both ways bi-amped)
I found good passive 18db xovers on the upper bass/ mids and highs of my Martin Logan ESL Monoliths III sounds more natural than any active crossover I tried, they all had a sterility to sound.
But on the 12" bass 150hz down I use and my own 24db active.

Cheers George

My favorite speakers of all time....I remember them fondly.