The land becomes too valuable to continue as relatively inexpensive residential.
I can't remember the exact percentage, but a relatively high percentage of Allston properties are owned by absentee owners. And the percentage in sub-neighborhoods differs. Absentee owners usually have no sentimental attachment to the property they own.
IIRC, Harvard has said it won't buy more property in North Allston for the foreseeable future, --there are probably several exceptions to that, one being the three story apartment house on Western Ave., east of Barry's Corner, which would represent infill purchases. But Harvard's promise doesn't forestall savvy investors from snapping up property as it comes on the market, holding it until it greatly appreciates in value, and then selling it.
If one drives down Route 127 in Beverly, that's probably a textbook case of how two schools, Endicott College and the Landmark School, have progressively expanded their campuses by buying property that was not cheap.
http://map.endicott.edu/
http://www.landmarkschool.org/downloads/hs/HSmap.pdf
Everything can be gained for a price.