I haven't been following much recently, but this is my understanding about how things stand:
1) Dark-Alex almost immediately mentioned that he wouldn't bother with the incremental updates such as 5.02 and 5.03. When 5.02 hit with updated PSN security, he released an incremental update to 5.00 M33-x that restored PSN access to us, without the need for a 5.02 M33. Even with 5.50 OFW out and about, we can still access the PSN with 5.00 M33-6.
2) 5.05 was never touched because it was a UMD-only release, and was never distributed to all users at large. Also, I believe it's just another very minor update, in any event.
3) Not long after 5.50 hit, Dark-Alex spoke up saying that the encryption keys were finally changed, and that current dumpers would not work on 5.50. That said, he also added that it wouldn't pose much of a barrier and that the development of a new dumper shouldn't be too complicated.
This was some time ago, so I'm thinking it's really more lack of interest rather than anything else that's delaying a 5.50 M33 update.
