I hope this isn't the real reason, but every engineer at Google has access to free and practically limitless CPU time quota, while GPU quota requires dedicated

I hope this isn't the real reason, but every engineer at Google has access to free and practically limitless CPU time quota, while GPU quota requires dedicated machines and therefore budget. It might have just been easier to do it on CPUs. Still 2500 CPU-hours is basically nothing. That's how much time a single machine with 2 sockets and 112 threads yields in a single day.