why the name of the family of grasses was changed from Gramineae to Poaceae?

Answer 1

The answer is that it wasn't really changed at all. Plant family names are usually made by taking the stem of the name of the "type" genus and adding '-aceae', the standard ending that indicates family names. In this case, the genus is Poa. The stem is 'Po-', add the standard ending and you get Poaceae.

However, a few of the larger, well-known families had traditional names that were based on a quality they possessed. Gramineae is taken from the Latin word for grass "gramen" (stem gramin-) plus an ending '-eae'. The rules for naming plants (the International Code of Botanical Nomenclature) gives both forms equal validity in these well-known families.

So, the answer is that they name was never changed but that the preference of users has now swung away from using a mixture of name formations to only using systematically formed names. Preference (you could call it fashion) is what makes the difference.

If you want to see the relevant part of the Code, see Article 18.5 at the link.

Answer 2

First the names have to be synomems with the Order they are associated with. The order for grasses is Poales and so the family was changed to Poaceae. The other factor was having the correct suffix of -aceae. As you can see Gramineae does not follow this ending.

This change is due to shifting to the "International Code of Botanical Nomenclature" which is based off of Evolutionary relationships.