The last time a 1st edition Base Set Charizard received a grade of PSA 10 was in March 2019. Additionally, only 2 have graded as a 10 within the last 2 years. (Source)
I think this is a milestone of sorts that really speaks to how difficult this card is going to be to acquire in the years to come. The total population remains at 120 but the demand is insanely high, even extending to the world of collectors who don’t typically collect Pokemon cards.
I have noticed a lot of loose 1st Ed Charizards making there way onto Ebay over the last couple of weeks. Most seem to be no better than a PSA 5 but they command a decent price.
I think the PSA 10 set is at such a high price point, that many people don’t see it as possible to collect a $100k+ set or if they have $100k+ to spend then they may be spending it elsewhere. Most of the WOTC cards increasing in price were in the $25-$1000 price range. That still doesn’t explain why the PSA 9 set has been stagnant.
I wouldn’t be surprised to see 1st ed base keep going up even more now that a lot of the other WOTC sets have spiked up substantially. Buyers may end up looping back around to 1st ed base as the price gap between base and the other WOTC sets has fallen in a lot of cases. As @charizandrew stated above though, the high price point may still be a barrier to entry.
Like others have said 1st edition base is already higher in price leading to being out of the price range of most collectors. However, many other cards are approaching 1st edition base prices, so I do believe 1st edition base will some large increases in the next 1-2 years.