The answer is that they felt "traditional forms of art, literature, religious faith, social organization, and daily life were becoming outdated in the new economic, social, and political conditions of an emerging fully industrial world."
The underlying principles of modernism are:
1. expression of volume rather than mass
2. balance rather than preconceived symmetry
3. expulsion of applied ornament
A great example of this is through Charles Le Corbusier, a great leader of this movement. In his design in the Villa Savoye in Paris we see this importance of modernism played out.
The five points are: 1) the pilotis elevating the mass off the ground, (2) the free plan, achieved through the separation of the load-bearing columns from the walls subdividing the space, (3) the free facade, the corollary of the free plan in the vertical plane, (4) the long horizontal sliding window and finally (5) the roof garden, restoring, supposedly, the area of ground covered by the house.
As one can see, the use of achieving modernism is of great importance in the beginning of the 20th century as in the same way the "modernism" in everyday things today changes, because it is thought to be traditional and out of date.