What are the technical reasons that BIP33 was never implemented ?
It seems to have expired
BIPs should be changed from Draft or Proposed status, to Rejected status, upon request by any person, if they have not made progress in three years. Such a BIP may be changed to Draft status if the champion provides revisions that meaningfully address public criticism of the proposal, or to Proposed status if it meets the criteria required as described in the previous paragraph.
To see the exact details of the discussion of this proposal I believe you need to become a member of the bitcoin developers mailing list and bitcoin developers forum.
If you look at https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2012-May/subject.html it seems this BIP was maybe superseded by some work on Bloom filters?
To get a complete picture, I think you'd have to dig around a lot more.