Well, only very specific viruses entry the nucleus, most often due to not bringing along enzymes needed for replication. Then, there's transducing viruses and non-transducing viruses, of which only one will retain it's genome in the host genome. Both occur near oncogenic gene locations too, which is why viral infections can lead to cancer, but this isn't very common at all. IDK, guess I've always felt bacteria are way more complex and that it makes sense CRISPR comes from them. Bacterial viruses, aka bacteria phages, were the evolutionary pressure which lead to CRISPR's development. But I'm a nerd and stay up to date with it all, so maybe that shifted my outlook.