I have answered this same question several times in he past, but once again, here it goes:
For TRIACS, you require phase control, not PWM control.
Phase control is in essence a monostable which is triggered by the AC waveform's zero crossing. In other words, synchronized, which your astable circui is not.
Since the firing phase angle is delayed from the zero crossing, you also cannot utilize an optotriac which only fires at or very close to the zero crossing. You require a random firing optotriac.