It really all comes down to being forced to accept things they didn't like. The things they were forced to do were:

Hold an asset they didn't understand.
Transact according to rules they had no say in.
Rely on third parties they had no contract with.
Have no guarantees regarding reliability.
Have little or no say over the way the rules might change.

And there were a lot of objections relating to the specifics of particular proposals.

For example, for bitcoin, many didn't like the involvement of the Chinese. For all they knew, North Korea might have a major investment in mining. And, of course, Ripple couldn't guarantee that people wouldn't trust validators run by North Korea.

The particular proposal Ripple was making used on-ledger, non-XRP assets. The major specific objections to this system largely boiled down to its complexity. Having to mirror part of your private ledger into a public ledger coupled with the uncertain legal status of obligations on the ledger was a problem. Did RCL actually represent a legally valid debt? Or was the debt a separate thing from the ledger entry? Could they legally hold a volatile asset like XRP, even to pay fees? And so on.

Many of these objections have faded over time. Some of them are still just as prevalent today as they were then. ILP answers almost all of these.