It's The Simpsons Syndrome all over again: a show that was great in the earlier seasons (in this case, primarily Season 1-3, which I rate as among the greatest put to television), entered a steady decline, and then jumped the shark. It's no great mystery; money has once again corrupted something good.
Like with the Simpsons, the show started off with a very solid base and great writing. It had to be well written in order for it to succeed. But as soon as it became a hit, the problems started. The first thing that happened was multiple guest appearances, largely for no practical or logical in-show reason (The Simpsons ring a bell?), which was then followed by changes in the writing to reach out to as many people as possible—demographics. You see, as soon as a show, or computer game franchise, or ANYTHING becomes famous enough, it can start branching out to more people in pursuit of more money. To do this, things have to change and, usually, these changes are for the worse. Greed gets in the way nearly every time.
In the case of TBBT, the changes were mainly designed to attract more female viewers.
The show's primary audience was originally sci-fi fans and technically minded people interested in science. News flash: that generally is not women. The show was funny because it had interactions between three desperate geeks, a genius with serious social problems, and their total opposite— a gorgeous female neighbour named Penny, who works at the Cheesecake Factory. A lot of people, male and female, enjoyed this dynamic, and there was a cleverness to the writing throughout. People more 'nerd' (a stupid word that insinuates being intelligent is a bad thing) oriented could understand the clever science references, as well.
That's all gone now. Beginning mainly Season 4 (although there were some silly individual moments beforehand), the show swamped the audience with brain-dead relationship story-lines, which a lot of women, sadly, find appealing. Also, dumb and relentless sexual references became the norm (in one scene, I kid you not, a main character goes to the bathroom to 'use her toothbrush'). 'Friends'-type crap is not what made TBBT great, but the writers don't care anymore. They want more money (and that includes the greedy cast. Look it up!). Sadly, like with The Simpsons, the show had no ending in mind when it started. There was no plan. Instead of a show that lasts a few seasons and has a respectable ending, we've got The Simpsons again—a show that just keeps on going, with no goal in sight, and with increasingly inferior writing to boot. I gave up with TBBT at the start of Season 9. Up until then, it was at least watchable. Now it's crossed over into insulting. The characters aren't even behaving remotely like they were in the first seasons, and the writers are waving a magic wand whenever they feel like it; for example, two of the main characters go from menial jobs to massive success in a very short period of time. But it doesn't matter, see. For every intelligent person that drops out, two status-quo loving, unimaginable dumb-asses have already taken their place.
This pattern happens an awful lot; there is nothing you can do except vote with your feet and watch the earlier seasons instead. It's sad that the characters were not done justice, all through the want of money instead of art. The show has become lazy, thick, and, even worse, predictable. It's over.