The major reason is probably cost.
Steel panels need expensive tooling to produce them, but once an investment has been made in that tooling the panels can be produced rapidly and cheaply. Drop in a flat sheet of steel and a few seconds later you have a shaped panel.
With GRP the tooling is relatively cheap, but it takes a long time to produce a panel from that tooling. The matting needs to be cut to shape then laid out (most likely by hand), with overlapping areas carefully aligned, all of which is a relatively skilled job. Some sections will need the 'grain' of the matting carefully aligning. Resin can then be added. To minimise weight (and keep it fairly consistent) the panel will then needing bagging up to suck out the excess resin while it cures, all of which takes time.
Carbon fibre is similar but even more time consuming.
If you are making a few hundred panels then the tooling cost savings of GRP outweigh the high labour costs, but once you get to the thousands the labour costs become uneconomical. Hence GRP bodywork tends to be heavily used for limited production vehicles
I suspect another major consideration now for production vehicles is the ability to recycle GRP panels.
Note that even in the 1950s Lotus put in to production a car with a GRP monocoque body shell with no separate chassis.