First the obvious reasons

1. Most of the bosses are behind bars.
2. Although law enforcement surveillance may not be what it once was the risk of getting caught is still too great.
3. Too many informants. I think even if all the other bosses were out on the streets the trust level is not the same as it once was between families.

The not so obvious reasons
1. Is there really a need for a commission nowadays? If you think about the climate of that world in 1931 that threat of authority was needed to keep thing under control on the streets. Nowadays you don't have to worry about bodies in the streets or in trunks.
2. This is just my observation and opinion but why would a decision on a beef need to come from the commission when 2 captains who are out on the streets and know what is going on can have a sit down and settle things themselves without having to get the bosses involved. I would think that Captains today have more power than they ever had. Again that's just my opinion and perspective. I can see those in charge today being more willing to delegate than the bosses of the past.
3. Would the Bonanno's and Colombo's really have a substantial voice on the commission? I would think the Genovese and Gambino families (generally speaking) probably look at them like a total joke. I'm sure they have members and associates who do business with one another but do you think someone like Bellomo or Cali gives a damn about the opinion of someone from the Colombo's or Bonanno's? I would guess probably not much.