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).
Or because affricates are composed of two different kinds of sounds (plosives + fricatives).
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].