It's a bit of a complicated story.
SeaWiFS had a very limited capacity to store the full 1km resolution (LAC - Local Area Coverage) data on-board, but it did continually broadcast the signal via High Resolution Picture Transmission (HRPT) and any receiving station with the proper equipment could record the data. The "MLAC" data are an orbit base merge of the on-board recorded LAC and the HRPT LAC data.
The SeaWiFS mission was a somewhat unique one for NASA in that the instrument and spacecraft were owned by a private company and NASA had a contract to buy the data. NASA was not the only customer, and to maintain the marketability of the data, the HRPT transmission was encrypted. NASA made the unencrypted data it obtained publicly available, but only after a two-week embargo period.
As part of the contract, NASA was allowed to provide decryption keys to a number of receiving stations. Others purchased keys directly from the company (Orbimage, a subsidiary of Orbital Sciences Corporation, which became GeoEye, which was eventually bought by Digital Globe). While many of the receiving stations that purchased decryption key participated in the network that provided HPRT data to NASA, not all did, nor were all HRPT stations operational - or at least collecting SeaWiFS data - for the entire lifetime of the mission.
The original contract NASA had for the SeaWiFS data was for 5 years. This contract was extended a few times (one of those times was in 2004). The global ocean color landscape (or seascape if you will
) was evolving rapidly around that time...there were two MODIS instruments and the ESA MERIS instrument providing embargo-free data and so the number of active HRPT stations collecting SeaWiFS declined.
After the demise of the SeaWiFS instrument in December 2013, one final push to collect any SeaWiFS HRPT data that stations may have collected but had not provided to NASA was made. What we have now is the most complete set of MLAC that is possible. There may still be data out there that we don't have, but it is unlikely.
So, long story short (I know, too late...) the decline in the number MLAC data files you see for your region of interest is real.