Because this will remove intent and knowledge from consideration. Under the STOCK act, you have to prove they're trading based on non-public information. Under the new bill, they just can't trade stocks, period. Any trading is immediately an offense.
I don't know why anyone is talking about insider trading. I don't give a shit if the congresspeople do insider trading.
What I care about is that if congresspeople can trade at all, they have a conflict of interest in doing their legislative duties. They are incentivized to pass laws that will benefit corporations and make the stock market rise. That's what is unacceptable about Congress trading.
Banning trading totally is the only thing that matters.
https://en.m.wikipedia.org/wiki/STOCK_Act