Not lock down people's devices basically. [L]GPLv3 requires that people are able to replace the [L]GPLv3'd binaries in their own devices if they want to (so they actually take advantage of the code being FLOSS instead of some nice theoretical novelty - ie. it is nice that my router uses Linux and find it amusing that it comes with the GPLv2 printed out, but what is the point if i can't open it up and fix/change some things that annoy me?). Apple wants to keep the devices they sell you under tight control, so [L]GPLv3 rubs them wrong.
It might also be about patents since IIRC [L]GPLv3 requires that if you use a [L]GPLv3'd program and that program infringes some of your patents then you are not allowed to sue others over their use of your patents in the same program.
And so they avoid it.
Related: https://en.wikipedia.org/wiki/Tivoization
It might also be about patents since IIRC [L]GPLv3 requires that if you use a [L]GPLv3'd program and that program infringes some of your patents then you are not allowed to sue others over their use of your patents in the same program.