Early stage investing is the right mix of low cost and high yield. Most of the their startups are failures and duds but it doesn't matter because they don't cost that much and there are enough that do work out. Later stage investments are still quite risky but now you are losing millions when they fail. So you are more dependent on the handful that don't. Also, late stage investment in Silicon Valley means dumping money in real estate, very expensive executives, engineers, etc. Who then take the money and job hop to the next startup as soon as things start failing.
Now that interest rates are going up, the financial risks have increased and the follow up rounds are drying up a bit (less IPOs, series B, C, etc.). Meaning the cost is higher and exits are becoming a bit more rare and less lucrative. And also, there is less chance of offloading not quite profitable late stage investments to more conservative investors. A lot of these unicorns still fail. They just fail a bit later after early investors cash in. Somebody pays for that. Usually not the early investors.
They did not say how their late-stage investing turned out, but supposedly not as well, and that's why they won't do it anymore.