For the kind of dollar values most github supporters are working in (probably in the range of less than $1000/year) - a single yearly lump sum would be easier to expense directly on a corporate card rather than getting accounting involved monthly.
This has been my experience, too. I suspect it has to do with how budgets are allocated.
A large expenditure request follows a different path and will probably get seriously examined and specially budgeted for.
A small one only needs the approval of the department manager, and it will come directly out of the budget that's already been allocated. This means that it is an expense that manager will actually feel, and is more likely to simply be denied.