1. EPA killed the 2.5
2. The 3.0 was never the best replacement, the boats didn't like the extra weight (though by now many have found a way to get reasonable handling even on some of the smaller V's, but the 3.0's were happier on 'bigger' 21+ V's and tunnels/mod VP's
3. Center Consoles, Wake Boats and other larger boats gained popularity. Remember, in the 70's/80's and early 90's you fished from a small aluminum boat or a bass boat and you water skied behind a boat that was designed to make a tiny wake. Now it is the opposite and you can't even use your hydrostream on saturday.
4. Optimax' came around and while good motors, they began to get too complicated and expensive for the average shade tree mechanic. They also were a part of the too heavy for the smaller boats so time to move up a little bit problem.
5. Bigger forces bigger. Everyone moving to those wake boats, CC's and pontoons forced anyone who wanted to go fast comfortably into a bigger boat. In 2005-2009 timeframe I could enjoy a saturday on Lake St. Clair in my 24' boat. Today, under 30 and you're getting your ass handed to you all day long.