In all likelihood; from my research ... Haplogroup L is European / Caucasoid - and this may have been a minor haplogroup or a tagalong haplogroup among (pre-Indo-European) Europeans. Perhaps it was abundant and settled in Europe with Haplogroup G bearing peoples; alongside the E-V13 (and various other E1b1b) and I2 and I1 peoples. The reason why Haplogroup L is rare in Europe is because it was already quite rare among settlers, but Indo-European settlers further reduced Haplogroup L to a very small frequency.

The reason why haplogroup L is so low is because it probably followed Haplogroup G into Western and Central Europe. I would say the same thing for Haplogroup T prior to Indo-European conquest/settlement; even though haplogroup T is shown in studies and maps to be more abundant in Europe. (If you take my theory analogously.)

If you follow the paternal haplogroups timeline; L and T are (apparently) descendants of Haplogroup K-M9 and are relatives to each other. - This is the analogy.

Most of Haplogroup L in all likelihood; came from Neolithic farmers. While the majority of Europeans in Neolithic Europe (at this time) were haplogroup G.

So, my conclusion is that Haplogroup L is nothing more than a relative Haplogroup to haplogroup T and G. E1b1b and so-forth, and it arrived in Europe as a minor haplogroup; with those peoples - and is nothing more than a minor Neolithic farmer haplogroup.