To focus on the original question, what physics factors caused the shift in quantum cosmology
from String (to a significant extent) in 1996-1998
to largely Loop in 2009-2011?
I'll unfold the links I gave in the first post:
Here is the Inspire topcited ten quantum cosmo list for 2009-2011.
Notice that it is pretty much all Loop, Horava and Verlinde.
==Inspire quantum cosmology top ten 2009-present==
Cosmology of the Lifshitz universe.
Gianluca Calcagni (Penn State U.). IGC-09-4-2. Apr 2009. 21 pp.
Published in JHEP 0909 (2009) 112
e-Print: arXiv:0904.0829 [hep-th]
Pathological behaviour of the scalar graviton in Horava-Lifshitz gravity.
Kazuya Koyama (Portsmouth U., ICG), Frederico Arroja (Kyoto U., Yukawa Inst., Kyoto). Oct 2009. 7 pp.
Published in JHEP 1003 (2010) 061
e-Print: arXiv:0910.1998 [hep-th]
Loop quantum cosmology of Bianchi I models.
Abhay Ashtekar, Edward Wilson-Ewing (Penn State U.). Mar 2009. 33 pp.
Published in Phys.Rev. D79 (2009) 083535
e-Print: arXiv:0903.3397 [gr-qc]
On Inflation with Non-minimal Coupling.
Mark P. Hertzberg (MIT & KIPAC, Menlo Park & Stanford U., ITP). Feb 2010. 13 pp.
Published in JHEP 1011 (2010) 023
e-Print: arXiv:1002.2995 [hep-ph]
Entropy-Corrected Holographic Dark Energy.
Hao Wei (Beijing, Inst. Tech.). Feb 2009. 12 pp.
Published in Commun.Theor.Phys. 52 (2009) 743-749
e-Print: arXiv:0902.0129 [gr-qc]
Casting Loop Quantum Cosmology in the Spin Foam Paradigm.
Abhay Ashtekar, Miguel Campiglia, Adam Henderson (Penn State U.). IGC-10-1-1. Jan 2010. 36 pp.
Published in Class.Quant.Grav. 27 (2010) 135020
e-Print: arXiv:1001.5147 [gr-qc]
Loop Quantum Cosmology and Spin Foams.
Abhay Ashtekar, Miguel Campiglia, Adam Henderson (Penn State U.). IGC-09-9-1. Sep 2009. 11 pp.
Published in Phys.Lett. B681 (2009) 347-352
e-Print: arXiv:0909.4221 [gr-qc]
Entropic cosmology: a unified model of inflation and late-time acceleration
Yi-Fu Cai, Jie Liu, Hong Li (Beijing, Inst. High Energy Phys. & TPCSF, Beijing). Mar 2010. 9 pp.
Published in Phys.Lett. B690 (2010) 213-219
e-Print: arXiv:1003.4526 [astro-ph.CO]
Towards Spinfoam Cosmology.
Eugenio Bianchi, Carlo Rovelli (Marseille, CPT), Francesca Vidotto (Marseille, CPT & INFN, Rome & Pavia U. & INFN, Pavia). Mar 2010. 8 pp.
Published in Phys.Rev. D82 (2010) 084035
e-Print: arXiv:1003.3483 [gr-qc]
Possible observational effects of loop quantum cosmology.
Jakub Mielczarek (Jagiellonian U., Astron. Observ. & LPSC, Grenoble). Aug 2009. 11 pp.
Published in Phys.Rev. D81 (2010) 063503
e-Print: arXiv:0908.4329 [gr-qc]
Numbers 1 and 2 are Horava-Lifshitz anistropic.
Numbers 3, 6, 7, 9 and 10 are obviously Loop
Number 5 is less obvious but if you read it you find roughly half is about Loop.
Number 8 is Verlinde entropic force.
Number 4 is just conventional straight QFT, no extra dimensions.
NONE of the top ten quantum cosmo papers here involve extra dimensions in any essential way.
Here are 18 "string model" quantum cosmology papers that appeared in the 1996-2000 period.
Eleven of these are 1996-1998:
Here is the quantum cosmology top 10 for the earlier period 1996-1998
It has three STRING quantum cosmology papers! If you wrote a string QC paper back then there is a very good chance it made the QC top 10 list! So things were different then.
There has to be some explanation for this change.