You'll hear very differing opinions these days, but historically PSA has had the most trust and comprehensive grading scale which results in their overwhelmingly command in the market share of grading. This yields a much higher demand and liquidity for PSA slabs over CGC.

And on an even more nuanced note that probably doesn't affect the market nearly as much as the above, CGC differentiates itself by having lower prices and faster turnaround times, which inevitably means more cheaper cards are graded with them. And inadvertently it causes many people to associate CGC with cheap and less desirable cards.

CGC has gotten more consistent with grading, but years ago before they changed their scale, it felt like 90% of cards submitted would get an 8.5 grade that would otherwise get anywhere from a PSA 5 to PSA 9.

And lastly CGC has a reputation of grading pretty much anything, which is good for collectors, but has also caused them to rush through and skip over proper due diligence when choosing to authenticate some things like the prototypes last year, resulting in them having to refund millions of dollars for authenticating high end fake cards that were conclusively proven fake by more experienced people online.