Not sure I believe that without evidence. Board games seem to have no trouble getting players to learn new rules for different games. Unless RPGers are very different in their willingness to learn. Could be wrong.
Short of some wargames, RPGs tend to be exponentially more complex in rules than most board games. Most commercial board games have rulebooks that are sometimes one to four pages in length.
and some wargames use the same, or very simmilar system or format from one to the next. SPI is the posterchild for that. Every one of their games that I have has the same general format, layout and even sometimes mechanics.
That familliarity helps for complex games and SPI games tended to be that as example.
Its a discussion that pops up with publishers and designers every few years and the end result is allways the same. Some prefer a core system and build from there. Some have incompatible systems for every product.