Cost, laziness, it requires a lot more skill than running a witness (nowadays you can get completely configured witness node, and run it with 2 commands)...
pick one :)
but yeah it is still way more expensive than just running a witness node, at some point we had to drop curie's node due to price and all other costs we had, but it shouldn't be much of an expense for people in top20 that are dedicated and know what they are doing. :)