From the T-Mobile FAQ at https://sites.google.com/site/tmoipv6/lg-mytouch

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4. My phone is not listed above, will it work with IPv6?

No, most phones do not have the Android radio firmware (RIL) that allows the phone to support IPv6 on the mobile interface. T-Mobile USA is encouraging all handset phone manufacturers to support IPv6. If more phones become available, we will update this site.

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It's mostly because of legacy. Your smartphone has (atleast) 2 operating system, there's the visible one (Android/iOS), and the one running the base band. The protocols used to talk to the cellular network is quite big, much, much bigger than just a TCP/IP stack, and there's usually more than one (often 2 or 3 of GSM, UMTS, LTE, CDMA). That part of the cellphone has evolved over the years, and when cellphones were able to connect to the internet over the cellular network, many stuck a TCP/IP stack in there as well, together with all the other protocols running on the baseband processor, and that decision has stuck. All this is abstracted away in android in the RIL, and while many(most?) Android devices does use the linux IP stack (as this picture conveys; http://www.kandroid.org/online-pdk/guide/telephony.html), not all do, and not all phones run Android - and are still leveraging the IP stack in the baseband. Cause it works, and it cost money to do something about it.