I got Shanghai-ed to push one of those amplifier carts in 1982, when I was the equipment manager/truck driver for the band - they couldn't get anyone else to do it!
I think 1980 was the first year we had the synthesizer, and I think the bass was added in 1982. I don't know when they stopped using it. Why did they stop using it? Probably because the amplifier carts were so heavy, and were not very mobile. Each cart had a large Peavy amplifier with two 15" speakers, four car batteries, and an inverter. It likely also was that the synthesizer just became obsolete, and they decided to stop using it, rather than invest more money. And it also likely coincided with the drum line starting to do actual formations, not just marching forwards & backwards between the sidelines. That's all that the carts did as well (go forwards & backwards).
The halftime show in 1981 was fun - we did an arrangement of Bach's Toccata & Fugue in D minor using the synthesizer. A Bruce Hall friend, and one of the original "Zebras", Mike "Spunky" Brunone, was the keyboard player that year.
With all of the mallet percussion, tympani, etc. in front of the band, sure, why not bring it back? - without the marching!