Why hasn't somebody invented a bunk and roller combo trailer?

It would be designed so that the boat sat on bunks, but a series of
rollers set alongside the bunks could be raised (hydraulics?) to ease
loading and unloading. The rollers would only need to raise enough so
that the tops were maybe 3/4 of an inch above the bunks. Getting the
rollers too high would defeat the purpose of allowing easier launching
and recovery with a bunk trailer. Best of both worlds, better support
for the hull on the trailer and easier launch and retrieval.

Would probably add a couple of thousand dollars to the cost of a boat
trailer, but I'll bet there would be folks who would gladly pay that.

Sportsman Trailers in Houston has been building them for years. You
pull a large lever (about 3') near the tongue and it raises a set of
rollers to lift the boat off the bunks. When loading you raise the
rollers, winch the boat to the stops and lower the rollers setting the
boat back on the bunks. I had one on an Aquasport and it worked great
- easy to load/unload single handed. I put an electric winch on the
trailer - I could walk to the end of the trailer, snap the hook to the
bow, engage the winch and just guide the keel onto the first roller.
Once it hit that roller it didn't matter what angle the boat was with
regards to the trailer - it would straighten and roll right up the
trailer - wind, waves etc. were no problem. Never tried driving on
since it was so easy. Their web site is
http://www.sportsmantrailers.com/ but they don't show the option even
though they still build them. Newer drop center drive ons have
diminished the market.