Maybe some of the founders are hoping that XRP will become the currency of the future and hoard it - but that would mean killing their company by pricing transactions to high.
This seems to be nothing but speculation. There is no clear reason why transactions would be too high just because the owners kept all their XRP, unless you think that the rest (50 billion) would be consumed very quickly. If every transaction costs a thousandth of an XRP, there would be a total of 5 * 10^13 possible transactions even without the founders' XRP. Transactions would be expensive if the supply of XRP were scarce, but it wouldn't be scarce just because the founders kept half of the supply.