(answer: probably because relapses and white matter lesions have been used to prove efficiacy in DMDs--even though atrophy, disability and disease progression continue.)
This answer reminds me of a Betaseron mini presentation that I attended back in 2000. The neuro, equipped with all his charts and paid by the drug company sponsoring this event, showed everyone in the room how the MS patients in the trial had fewer white matter lesions and relapses than the placebo group. In the question period afterwards, I stood up and asked him if any of the patients on the drug actually felt better for taking the medication. His muted reply...No! You could have heard a pin drop in the room where some 100 people sat. And the look I got from him was less than cordial.
At the same time I asked why the MS docs had no interest in doing any work on Prokarin which was just surfacing as a possible alternative medication for MS symptoms. The neuro had some nasty things to say about the Prokarin. After the presentation the pharma rep came over to me and started to ask some questions about the Prokarin. I asked her why the docs had absolutely no interest in checking out the Prokarin and she leaned over quietly and whispered into my ear..."because nobody will cross their palms with some cash to do it"!!
And this was but another incident in my long history in following the world of MS medicine.