Oftentimes gliders are not equipped with transponders (at least in Europe). Even if they are, they do not necessarily operate it at all times. This is allowed by SERA.13001:
(a) When an aircraft carries a serviceable SSR transponder, the pilot shall operate the transponder at all times during flight [...]
(c) Except for flight in airspace designated by the competent authority for mandatory operation of transponder, aircraft without sufficient electrical power supply are exempted from the requirement to operate the transponder at all times.
Modern gliders give a bad target for primary surveillance radars, because they are small and contain little reflective material. Thus gliders maybe unknown to air traffic controllers and flight information services. A transponder would give ATC secondary surveillance radar data which could help to avoid mid air collisions between gliders and other planes, especially airliners which may be too fast for practical "see and avoid" (as e.g. demonstrated in the event of an airprox between a glider and an A321 near Hamburg, Germany).
I could not find definite sources about the power consumption of transponders, however this question implies an average consumption somewhere between 2W and 6W. Considering that USB power banks with a capacity of 100Wh are small, light, cheap and could easily provide the energy needed for >16 hours (a time I expect should cover >99.99% of all glider flights) it seems that technically the power supply should not be the reason for not operating a transponder.
Another reason may simply be money. A quick google search implies about 2000 € for equipping a glider with a transponder which seems quite a lot considering that this is quite a simple device (e.g. compared to a smartphone). Nevertheless it is quite an easy measure to increase flight safety.