To put it brutally, because the outbreak isn’t that bad at present.
Yes horrible for those concerned, but a feature with Marburg outbreaks is that they tend to run out of steam before becoming widespread.
Something like 70% of cases in this outbreak are among HCPs, who might have been in close contact with patients before anyone realised that Marburg was the cause (early symptoms can be non-specific; the onset of haemorrhagic ones obviously triggers far greater concern, and by then it won’t take long to distinguish which it is).
So I expect that the precautions against spread will have been in place for over a week by now.
What do you think Rwanda needs - its public health surveillance is working well (this was spotted early, measures are in place)
The fatality rate varies enormously between outbreaks anyhow - as they tend to be small so the outcome of a few cases can make all the difference between the lowest ~28% to the highest ~85%
It is worse amongst the malnourished (as are many diseases) and poverty is a key factor. Poorer medical facilities is a proxy for areas with high poverty and malnutrition. We can’t fix that by sending supplies
What we could do, if the situation worsens, is what we did in Sierra Leone when Ebola struck in 2014 (overwhelming the local infrastructure with something like 10,000 cases) and send the army to build and staff treatment centres.
Sierra Leone was however part of the British empire, and British troops had last been deployed there as recently as the early 00s (during the civil war)
Rwanda was never part of the British empire, so I would not expect UK to have a leading role in the same was as for SL (though I expect we’d contribute is asked). Main role might be better taken by the Belgians