Any resident (ex-)tankies can confirm or deny this but I believe that, even with external bins/baskets, only so much stuff can be carried in them &, even then, that is usually reserved for essential equipment which needs to be kept protected from the elements. Personal gear, extra supplies (both issued & appropriated) & spare MG/AGL ammunition are carried externally to reduce the fire risk inside the vehicle by limiting the amount of combustible materials.
As to why the Americans tend to go for open baskets rather than enclosed bins; I have no idea, but it may be that US military thinking is that what is stored in the bins/baskets may need to be accessed quickly, possibly in combat, when accessing open topped baskets is easier than opening closed (& possibly latched) bins. Plus; closed-top bins limit the dimensions of what can be put into them, whereas open-topped baskets allow an increased range of dimensions for items to be stored. (PS: Bins with the lids removed/left open don't drain water as well as mesh baskets, so your gear ends up floating if you do that.)
