Because these two types of remittance will certainly break their own business if they are going to accept bitcoin.
And if ever they will accept bitcoin, it means that they surrendered to bitcoin. Just look at PayPal, though they are aware about bitcoin's potential and success.
They still don't adopt it
There may be plenty of reasons why they don't accept or adopt Bitcoin
I have already mentioned it before but it seems to be worth mentioning it again. PayPal just like other remittance companies such as Western Union or MoneyGram is working in many countries across the world, but Bitcoin regulation differs between countries dramatically. In some countries it is perfectly legal and welcome (say, in Switzerland, known for their love toward hard currencies) while in others it is outright prohibited (say, in Iceland), so these services may run into serious legal issues when sending bitcoins to the latter countries. It is like weed in the US, in some states like Washington it is legal, in some others like Indiana it is illegal
Yes I did mentioned that already but you are getting far from the whole topic we are just talking about moneygram and western union only here.
But your comparison seems to be true and I find it right. I really can't see that they will accept bitcoin in the future.
Or else their business will be dead.