Because they (LATCC and their CAT) didn't need it, and we (certainly gliding, and maybe other GA) did.
At least one bit of LTMA should never have been CAS in the first place - it was not required to accommodate any procedural things such as SIDs or STARs, and was a flagrant breach of evidence given by the CAA at the 1983 Stansted enquiry that they never took a single cubic centimetre of airspace that wasn't needed. The CAA had told me soon after it was first grabbed that it had no justification, but it was too much hassle to release it again very quickly. It took years of nagging before they finally let it go again.
It is things like that that make the BGA fight very hard over every piece of new CAS, unless the justification is strong.
Chris N