The 741 is an inferior opamp that is not capable of reaching Vcc or ground at its output. You can solve this by using a different op amp, such as types advertised for "rail to rail output" for example, or any op amp that is rated to swing within 0.3 volts of V-. Otherwise, you live with a low output of 1.5 volts and add an additional transistor stage to translate this to ground. Or you could change the way you attach the optoisolator to tolerate a low of 1.5 rather than 0 volts.
The ability of an opamp to swing to V- is clearly stated in their data sheets. For example, the National data sheet for the LM741 calls the parameter "output voltage swing". The value given is +/-14 volts typical if the load is 10Kohms or greater. This tells us that if the 741 power supply is +/- 15 V, the output can reach -14 volts at the low side with light loading and about -13 volts under heavier loading. This difference between -15 and, say, -13 exists no matter what voltage you bias V- to. So if V- is set to 0 volts, then the output can only swing down to 2 or perhaps 1 volt. In your case, you got to 1.5 volts. Quite predicable using the data sheet info.