A space elevator is a well established concept.
Despite being proposed for the first time in 1895, it has not yet found concrete implementation. Why?
Basically we have not yet found any material which is strong enough to withstand all the forces involved in the utilization of a space elevator.
A space elevator cable would need to carry its own weight as well as the additional weight of climbers. The required strength of the cable would vary along its length.
The cable would need to be made of a material with a large tensile strength/density ratio. For example, the Edwards space elevator design assumes a cable material with a tensile strength of at least 100 GPa. Since Edwards consistently assumed the density of his carbon nanotube cable to be 1300 kg/m^3, that implies a specific strength of 77 MPa/(kg/m^3). This value takes into consideration the entire weight of the space elevator. An untapered space elevator cable would need a material capable of sustaining a length of 4,960 kilometers (3,080 mi) of its own weight at sea level to reach a geostationary altitude of 35,786 km (22,236 mi) without yielding. Therefore, a material with very high strength and lightness is needed.
For comparison, metals like titanium, steel or aluminum alloys have breaking lengths of only 20–30 km (0.2 - 0.3 MPa/(kg/m^3)). Modern fibre materials such as kevlar, fibreglass and carbon/graphite fibre have breaking lengths of 100–400 km (1.0 - 4.0 MPa/(kg/m^3)). Nanoengineered materials such as carbon nanotubes and, more recently discovered, graphene ribbons (perfect two-dimensional sheets of carbon) are expected to have breaking lengths of 5000–6000 km (50 - 60 MPa/(kg/m^3)), and also are able to conduct electrical power.
As of today many people are betting on carbon nanotube or graphene as material for building the cable. If they don't manage to satisfy these expectations, well, you have your explanation on why the space elevator cannot be realized.