It is likely a cost-benefit reason for the use of nitrogen in this particular purpose. Nitrogen is easily the most available mostly-inert gas. Another common alternative is Argon, which is widely available in the atmosphere as second best to Helium. Helium however is becoming scarser, making its general use more and more restricted. However, as you say, Helium would be the optimal gas used for this type of purpose.

The nitrogen molecule is larger than that of helium and has a higher heat capacity, it is able to draw more thermal energy from the purged solution. yes, Helium can also be used for conservation of Meteorites. Argon is the cheapest noble gas and thus the most frequently used. Nitrogen also easily available and used but disadvantages it heavy and or human used it is replaced oxygen when used.

A problem could be due to the atomic radius of helium which is much smaller than the molecular radius of nitrogen: helium therefore permeates the rocks considerably, especially if the exposure lasts a long time. Doing so could alter isotopic investigations within the rock itself.