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 because affricates are composed of two different kinds of sounds (plosives + fricatives). 
<br>We usually geminate two similar sounds when they're next to each other:

- *Bad day* -> [bæd.deɪ]
- *This sin* -> [ðɪs.sɪn] etc.

The /d/ and /s/ can be geminated because we don't have any complex segments here. 


However, when two affricates come next to each other, we get four different kinds of sounds:

- *Orange juice* -> [ɒɹɪnd͡ʒ.d͡ʒuːs]: [d ʒ d ʒ].

- *Which chair* -> [wɪt͡ʃ.t͡ʃeə]: [t ʃ t ʃ].