Because Boeing's production lines are full?
Unless Airbus invests a ton of money in A350F to improve its abilities I don't see it selling that much at all. It will have to compete with cheap end of line 777F and Boeing 777ERSF.
A330 NEO-F is not launched and may never see the light of day as 330 goes the way of the DODO.
Cargo doors exist (Boeing 737-800BCF) that can be easily be adapted to the MAX.
Why would Airbus have a development cost advantage on Freighters?
It’s not so much an advantage as it is an opportunity. As I posted a few pages ago the A350 became profitable to produce back in 2019 and with around 500 still to be delivered it’s going to make Airbus a lot of money with or without a freighter version. We also know a freighter variant was planned from the outset, so it’s safe to assume that it is going to require minimal effort; expense and time to put it into production.
There are also a few other factors which I believe only re-enforce what a great opportunity this is for Airbus, consider;
From the 200+ orders for the 777F over the last decade we know that there is going to be a demand for a modern, regulation compliant replacement and If they are able to get a 2025 EIS it's possible they would have the segment all to themselves for a good few years given the 777X’s delays.
The A350F also has the potential to pave the way for a future A330NEO/F to be a regulation compliant replacement for the 767, considering that the NEO is already flying and the 330F already exists this should not be an expensive or complicated task for Airbus. The common type rating could also help sell either type to a current operator of either.
Finally, it’s also worth mentioning its potential to really kick Boeing when it's down, it’s not pleasant a pleasant thought as an enthusiast but it also cannot be ignored. Irrespective of written off costs the 77X is in a precarious position at present with delays and the inevitable late payments Sir Tim will be demanding not to cancel 50% of its order book Boeing really need a successful freighter variant to make the whole project worthwhile. Airbus on the other hand don’t, the freighter is a low cost, low risk endeavour, but it has the potential to not only open up an entirely new market segment but also make it very difficult for the competition. Even if they only sell enough to break even, it will be a win for them.