I guess there's a few reasons, and from a business point of view, I think it makes sense:
1) They've invested already in marketing and might have to pay the fighters their salary, or at least part of it, if they're forced to cancel the event. This way at least they can make some of that back.
2) Running an event with no crowd means you miss out on the live gate, but also probably helps cut many costs in security, cleaning, venue rental, making the loss from the gate not so bad.
3) Just running the UFC and paying its employess probably costs a bunch of money, so anything to offset that during these months helps.
4) They're probably expecting an uptick in buys since people are stuck at home and there's no competiton. Without Khabib, without bars showing the fight, without people being able to get together and split the cost, and with many people out of a job, I don't think it'll be much. But even if they do 200-300k buys (which I think is very low) at $70, that's 15-20million gross income.
5) There have been a lot of rumors that WME is not solvent, or is in somewhat desperate need of money, and the UFC is their biggest asset now, since their other businesses largely depend on live gates to generate income.
6) On a more speculative note, Dana might have taken on this project on a personal level. He's an extremely ambitious guy who'll do anything to get his and the UFC's way, and this is just the ultimate expression of that. This works for him on many levels, some of them beyond mere financial motivations.
