V838 Monocerotis "light-echo" images morphed into nice video, but why so few original images?
To get time on the Hubble Space Telescope is a difficult, peer-reviewed process. The time is usually about ten times oversubscribed. The scientific justification to get more images must be more than "it would make a nice movie".

The submission of proposals for HST time is a confidential process; so I don't know whether the principal investigators for these images asked for more at different epochs and were turned down, or whether they thought this many images were sufficient for the science they wanted to accomplish.

There is no technical reason why they couldn't have obtained more images. In fact more images were obtained (see this link to the Hubble archive) at later epochs between 2005 and 2012. A problem I guess with incorporating these images into the movie sequence were that the light echo gets fainter, more diffuse and starts to exceed the size of the detector, which would necessitate more than one pointing. They were also taken with a different camera, which might raise aesthetic compatibility problems. I show below a fairly crude colour composite I extracted from the archive for an observation in 2007.