Full Idea
The two main motivations for nominalism are an admirable commitment to Ockham's Razor, and a queasiness about postulating entities that are unobservable or non-empirical, existing in a non-physical realm.
Gist of Idea
Nominalists are motivated by Ockham's Razor and a distrust of unobservables
Source
J Hoffman/G Rosenkrantz (Platonistic Theories of Universals [2003], 3)
Book Reference
'The Oxford Handbook of Metaphysics', ed/tr. Loux,M /Zimmerman,D [OUP 2005], p.56
A Reaction
It doesn't follow that because the entities are unobservable that they are non-physical. Consider the 'interior' of an electron. Neverless I share a love of Ockham's Razor and a deep caution about unobservables.