The main reason is that *gemination* does not take place in complex segments. 'Affricates' are complex segments; they start off as plosives, but finish as fricatives (they have two manners of articulation).

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Or it could be that the gemination of 'affricates' usually results in a geminated *plosive* rather than a geminated affricate:
<br> For example, [t͡ʃ:] (or [t͡ʃ t͡ʃ]) is usually realised as [t͡:ʃ] (the plosive being geminated). 

*Which chair*: [wɪt͡ʃ.t͡ʃeə]. 
<br>If you geminate the [t͡ʃ], then the pronunciation is likely to change to [wɪt͡ːʃeə] which is almost the same as *wit chair* [wɪt͡ːʃeə].


If there were gemination in *orange juice*, it would most likely be pronounced [ɒɹɪnd͡ːʒuːs] which is almost the same as *orind juice*.