Yeah, I remember El-cassette! As I recall, the cassettes were about the same size as today's VHS cassettes, and I'm sure that some of the VHS tape technology derived from this extinct format.
There were three advantages to the El-cassette vs. regular cassette:
1) Wider tape, which was also a 2 track recording (only 1 side, couldn't flip them over) NOT 4 tracks allowing for higher recording levels, (less noise).
2) Faster speed (2x) for higher frequency response.
3) The entire tape drive was handled by the machine, NOT inside of the cassette! In an El-cassette, the tape was pulled from the cassette, and tape guide components were in the machine. With a conventional cassette, the tape drive path is handled inside of the cassette. This can result in huge azimuth alignment problems, with a severe loss of high frequencies! The cheap plastic shell supports the entire tape guide path, and different brands, manufacturing lots, or temperature variations can all affect azimuth! And azimuth alignment even varies when placing the same tape in different tape decks! Nakamichi was the only manufacturer (to my knowledge) that offered an azimuth adjustment!
Pre-recorded cassettes are usually duped not in real time, but at several times actually speed. The high frequency response goes right in the crapper. Pre-recorded El-cassettes would not be cost effective, since they would have to be duped in real time or to develop a duping machine, running at least 120 IPS, which may be an impossibility. Running an El-cassette at 30 IPS, with a conventional non-rotary head, would limit recording time to about 10 to 15 minutes. The transport and braking motors needed to run at 30 IPS would make an El-cassette machine excessively large, heavy, and expensive, if at all possible. It is interesting though, that I have an older Mitsubishi Super VHS machine (U-71), from the early '90s. One of the applications listed in its manual, is using S-VHS tapes for high fidelity audio only recordings.
When all is said and done, the old method of mechanical analog recordings (dating back to Edison's wax cylinders) may survive all of the newer formats. Now that's going full circle!