I think there are several reasons for this. I also think the reasons are often individual.
One of the issues at this time, is the change in the whole MOTU system. I believe, the changes can be confusing, and have been implemented more on technological lines and unfortunately did not bring the community fully on board (maybe just because it is confusing).
I also think, in some cases the motivation to be a MOTU is not as clear as it could be. IMHO, being a MOTU is a responsibility, not a privilege. It is not about the title, but about the ability to help the Ubuntu community by the access rights that come with it. Due to this, it could be that the whole approval process could be modified (or extended). MOTUs usually nominate themselves, and then the board looks if they are ready to be MOTUs. Maybe it should be possible, that peers that believe that someone is ready to be a MOTU be able to nominate that person. This would IMHO represent more the fact, that the nomination is done to help the process, not to obtain a title. I understand that making this the sole way has its problems too, therefore, I rather see it as an alternative then the only way.
I also know there have been some problems in the past with people focusing more on KDE. These problems have hopefully been addressed, but maybe it would good if that would also be more widely known.
Obviously, these are just a couple of issues that I have notice. People are different and will see different things, or be affected differently by the same thing. So, theses issues might not stop everyone, nor are they the sole reasons for this problem.