What I don't understand is why anyone would send a transaction in Bitcoin, which is slow and expensive compared to say other reliable coins like my favourites #gridcoin or #solarcoin, which are much faster and have far lower fees (even when exchange fees are added).

Network effect, network effect, brand name and network effect.

"Everyone" has heard about Bitcoin, while even quite some insiders in the crypto-currency-community haven't heard about your favorite coins. It's needed with two parties to transact, and both parties needs to support the same coin. Even if both parties does support one other alt, the likelihood that both of them supports the same alt is small.

I believe that there are lots and lots and lots of ordinary folks that buys Bitcoin as an investment. Brand name matters; probably Bitcoin seems to be a much safer bet than some random altcoin.

And then the on-ramps and off-ramps matters too. If you have physical cash and want to exchange it into crypto currency, what would you do? Most likely you'd visit a site like Localbitcoins - and they serve only bitcoins, period. So even if you want to exchange your physical cash into some rare altcoin, you'd probably go via Bitcoin.

What I really don't understand is why organizations like OpenBazaar and BitPay still stick to their Bitcoin-only-policy. With payments being their bread-and-butter, it would most likely be prudent to diversify and give people options to use cheaper and better alternatives; particularly when even the current Bitcoin developers seems to think the Bitcoin is not for payments but for "store of value".