I'd imagine there are a few reasons, but the TL;DR is... money.

1. This year's April Fools event was terrible. Most people were unable to participate as they didn't understand it and those that did participate worked on the event in a couple of different Discord servers, not on Reddit itself (because Reddit is terrible for real-time communication). It was also largely unannounced so it was pretty challenging to find and that couldn't have been good for the site's revenue - as controversial as r/place is going to be this year, it's going to bring in a lot of money for the site (and increase the number of active users) because even people that have left Reddit will probably return to put "FUCK SPEZ" on the canvas.
2. The API controversy is ongoing and shows no signs of ending currently. I think they're trying to distract people from the API changes and hope that people think "wow, Reddit is fun, I'll stick around" after r/place ends instead of becoming less active or leaving the site entirely. r/place will also probably be discussed quite widely for the next few weeks and Reddit probably hopes that this will lead to the API changes being forgotten about (like plenty of other controversies in the site's history - newer Redditors have no idea about the r/jailbait controversy or u/spez editing comments, for example).

Ultimately, Reddit are going to be collecting data from r/place and using it to encourage investors to invest in Reddit.