Why don't 1,2-substitued alkenes polymerize as much as mono or disubsituted alkenes?
It is true that we do not see the products made by the polymerization of 1,2-substituted alkenes or 2-olefins (even natural rubber does not fall in this category). Although a number of 2-olefins such as 2-butene in large amounts is produced by petroleum industry (naphtha cracking), they have not been utilized as monomers to obtain commercial polymers in daily life because of their low reactivity toward commonly used polymerization catalysts, probably due to steric effects. Perhaps, if 2-olefins are reactive as much as ethylene towards polymerization to give high molecular weight polymers, there would be a large number of new polymer materials in daily use.
Nonetheless, a few studies on the polymerization of 2-olefins by novel transition metal complexes, specifically using trans-2-butene as the monomer, have been reported (Ref. 1-3). According to all these three references, the monomer-isomerization has been occurred during polymerization process, indicating the steric effect. For instance, both cis- and trans-2-butene give high molecular weight polymers with a recurring unit of the corresponding 1-olefins (after monomer-isomerization by metal catalyst prior to the polymerization), which consists of $\ce{–CH2–CH2–CH(CH3)\!–}$ units instead of $\ce{–CH(CH3)–CH(CH3)\!–}$ units (Ref.1 and 3). Thus, it is unlike that highly condensed polymer can be produced from 2-2-olefins until the discovery of a novel metal catalyst, which prohibit the in situ monomer-isomerization.
References:

Mark D. Leatherman, Maurice Brookhart, &quot;Ni(II)-Catalyzed Polymerization of trans-2-Butene,&quot; Macromolecules 2001, 34(9), 2748–2750 (DOI: https://doi.org/10.1021/ma002142j).
Mark D. Leatherman, Maurice Brookhart, &quot;(E)−(Z) Selectivity in the Polymerization of 2-Butene Promoted by Ni(II) Brookhart-Type Catalysts,&quot; Macromolecules 2005, 38(6), 2072–2075 (DOI: https://doi.org/10.1021/ma048373y).
Kiyoshi Endo, Yoshiyuki Kondo, &quot;Polymerization of trans-2-Butene with ($\alpha$-Diimine)Ni(II) Complex in Combination with $\ce{Et2AlCl}$,&quot; Polymer Journal 2006, 38(9), 1160–1164 (DOI: https://doi.org/10.1295/polymj.PJ2006058).