As measured by turnover at the top 10.
Looking at the top 10 (Champ+1 through 9) annual HW's from Ring magazine over the last 5 years, there have only been 16 top 10 boxing HW's for 2009-2013. For MMA, Sherdog's December rankings for 2009 through 2013 have a total of 17 distinct names, about the same. (I didn't see any truly apples-to-apples annual mma rankings, but the results seems right).
Since there's way more money at the top in HW boxing, why hasn't there been more competition as well? iI seems like the divisions everyone agrees are more competitive have a lot more turnover.
Maybe it's just really, really hard to get guys to fight big guys--even if they themselves are big--because fighting big guys is just plain terrifying for everyone.
A lot of rankings are US centric or US/western Europe centric and routinely overlook eastern europeans/former USSR country athletes. The rankers don't see a lot of the fighters or fights in those areas so they downgrade them. You can see a similar situation in the US itself, boxers from certain areas are talked about with the following disclaimer, he is 15-0 but "most of his fights were in North Carolina, or Minnesota or X state" because many view only a few states as having tough, skilled competition.
HW division in both sports are shallow because most people don't want to be punched in the face for a living.
In what way is this specifically related to the HW division?
Relation could be that the potential talent pool for heavyweight is already small; most athletes around the world are not heavy enough to compete in the heavyweight divisions.
As for American athletes, it isn't all about financial incentives. In the US there are many, many more youth leagues for other sports. Junior highs and high schools have field dozens of sports teams and very few offer boxing (some prep schools do have teams).
Look at the top high school sports for boys by number of participants for 2012-13:
1. Football - 1,115,208
2. Track and Field – Outdoor 580,672
3. Basketball 538,676
4. Baseball 474,791
5. Soccer 410,982
6. Wrestling 270,163
7. Cross Country 249,200
8. Tennis 157,247
9. Golf 152,584
10. Swimming and Diving 138,177
Other sports are more easily available to kids and they grow up playing them.
Take those 1.1 million kids playing football, estimate 1/4 of them graduate each year, so 280,000 potentially bound for college. Given the number of DI teams - FBS & FCS - there are only about 8000 kids going into DI every year. Obviously more than that go on to play DII, DIII, NAIA, juco but very, very few of those are going to make it to the pros (maybe a few make a NFL roster every year).
Also college football offers a massive number of scholarships compared to other mens sports: 85 total for DI/FBS and almost every school offers the full amount while many other schools don't offer the full number allowed for other sports even if the maximum allowed is 18, 14, 12.6, 9.9, 4.5, etc.
FCS football I believe allows 63 scholarships spread over a maximum of 85 kids.
DII football allows up to 36 scholarships; so even DII football allows more scholarships than every other DI sport outside of football (the next highest DI sport is hockey with 18 max). So it is common to see kids going to play DII football instead of a DI sport besides football.
Kids are playing those sports because they want to and they are easily available. After participating in them until they are 18 or early 20s, few guys want to make the transition to boxing or mma. You're talking about another few years of training before they are even going to be decent at it, for most that is a path they don't want to take.
Heck, the vast majority of collegiate wrestlers, the guys that have already learned a skill set to transition on, don't even consider going into mma. Look at some recent national tournaments and see how few actually are fighting.
Example: I think only 4 all-americans from 2011 (80 total) have had a mma fight.