Why do not become stable in arid climate? See the photos below of ephemeral streams in arid zones. They seem to be stable. However, statement may not be generalized. It depends on several factors. 

https://www.researchgate.net/publication/270489601_Aerial_Photo_of_Wadi_Main_stream_in_Arid_Region?ev=prf_pub

https://www.researchgate.net/publication/270490429_Aerial_Photo_of_Steam_Network_in_Arid_Zone?ev=prf_pub

Data Aerial Photo of Wadi Main stream in Arid Region

Data Aerial Photo of Steam Network in Arid Zone

Dear Oscar,

the reasons for lateral migration of meanders are certainly manifold. One simple answer to your question is that the river banks of meanders are stabilized by vegetation and the stabilization by plants is certainly more effective under humid subtropical climate conditions than in areas with dry climate. For further information see, e.g.:

Sean J. Bennett and Andrew Simon (2013): Riparian Vegetation and Fluvial Geomorphology (Wiley).

Cheers!

Elmar

Lack of riparian vegetation would certainly be a factor but the flashier arid regimes characterized by absence of precipitations/infrequent brief storms - as opposed to  more frequent and lighter rains - would also be decisive.

Based on these answers I make a suggestion of using the method of CI (Martín-Vide, 2004) and see if concentration of rainfall is there in the area.

http://onlinelibrary.wiley.com/doi/10.1002/joc.1030/pdf

Dear Oscar,

The reason are definitvly multiple but it seems that vegetation play a key role in fixing fluvial geomorphology as elegantly demonstrated by Gibling, M. & Davies, N. Palaeozoic landscapes shaped by plant evolution Nature Geoscience, 2012, 5, 99-105. (https://www.researchgate.net/publication/232752854_Palaeozoic_landscapes_shaped_by_plant_evolution).

Flash floods with very intense rainfalls also occur in subtropical climate, it tend sometimes to temporaly change the geomorphology of the rirvers, as long as vegetation come back!

I hope it will help you.

Best regards

Guillaume

Article Paleozoic landscapes shaped by plant evolution

Discharging groundwater can contribute to the stability of a meandering channel.  Many rivers meander simply because it is a more efficient way to drain adjoining aquifers and remove groundwater discharge from the landscape.  Without a humid climate to feed groundwater discharge, there is less need for the 'effective drainage length' that a single-thread meander channel provides (Pederson, 2009). 

Groundwater preferentially discharges along the concave banks of meander bends (Linderfelt and Turner, 2001).  Outflowing groundwater weakens the stream bank, making it more susceptible to lateral erosion by the migrating river.  Where groundwater discharge is an important factor in meander formation, streams can be pinned against the valley sides due to the larger, seepage-induced erodibility of banks aligned to the floodplain limits (van Balen et al, 08). This stabilizing “fixation effect” is likely to be more pronounced in humid regions, where groundwater flow systems are more active. 

The stabilizing effect of bank vegetation has already been mentioned.  High water tables can increase bank vegetation.

References:

Linderfelt, William R., and Jeffrey V. Turner. "Interaction between shallow groundwater, saline surface water and nutrient discharge in a seasonal estuary: the Swan–Canning system." Hydrological Processes 15.13 (2001): 2631-2653.

PEDERSON, Darryll T. "GROUNDWATER EFFECTS ON EVOLUTION OF ENTRENCHED CHANNELS CAUSED BY CHANNELIZATION." 2009 Portland GSA Annual Meeting. https://gsa.confex.com/gsa/2009AM/finalprogram/abstract_160651.htm

van Balen, R.T., C. Kasse, and J. De Moor. 2008. Impact of groundwater flow on meandering: Example from the Geul River, the Netherlands. Earth Surf. Processes Landforms 33:2010–2028.

Article Impact of groundwater flow on meandering; example from the G...