Because, the ability of broadleaved trees to absorb water through the leaves is less than 1%, and the ability of conifers to absorb water through the needles is 10% of needed water. That's mainly because of one small basal area on every needle that has no cutticle and has a strong absorbing capability.
Mugho Metuselah is sprouting very good.
Collection story: http://yamadoriarea.blogspot.com/2009/10/mugo-metuselah.html
and Constrictor...they are all very healthy. They flower and sprout new shoots readily... http://sandevbonsai.blogspot.com/2009/09/mugo-constrictor.html