Before I answer that question, let me first ask you: who do you think Thunderstrike was?
If that image leads to a response which is anything like "grim & gritty Thor," then you certainly never read the book. Indeed, it's surprising how frequently I see people make that description considering how conservative creators (and frequent collaborators) Tom DeFalco & Ron Frenz were and are. They usually aim to fashion comics evocative of Stan Lee & Jack Kirby, stories full of breezy, fun dialogue and dynamic, energetic layouts.
Eric Masterson was introduced during their Thor run as a new human counterpart for Thor, an everyman who became Thor's alternate identity and eventually was placed in command of Thor's power, something he never got the hang of. His story had been intended to run only in the pages of Thor, but the ever-expanding Marvel saw an opportunity to branch out, thus he took the identity of Thunderstrike in his own series (and soon found he had different powers he would fail to fully get the hang of).
As the covers proclaimed, Thunderstrike was the "Everyman Avenger," a fellow trying to hold down a career and make time for his son while facing a custody battle with his ex-wife (one Eric loses). As a hero, he had good intentions but despite his powers he could easily flummoxed by situations he wasn't prepared for. In one instance, his ex-wife's new husband Bobby had, while under the influence of drugs, locked himself in a room with Eric's son Kevin. Fearing the worst, Eric became Thunderstrike and came crashing in to save his son. However, Kevin (not knowing who Thunderstrike was) had already defused the situation with Bobby and chastised Thunderstrike for resorting to violence rather than trying to help someone who was, essentially, sick. Kevin summed up Thunderstrike as "a big bully."
But such bouts of anger were uncommon for Eric. Frequently, when faced with villains who could match his power (Juggernaut, Absorbing Man) he would seek a compromise rather than a confrontation, seeing little point in endangering peoples' lives in the mere hope he might win a fight. And ultimately, that was how Eric's story ended in Thunderstrike #24 - overcome by the dark power of the Bloodaxe and transformed into an enemy of the Avengers, Thunderstrike destroyed himself to eliminate the Bloodaxe's threat (essentially the Dark Phoenix Saga ending).
DeFalco has stated that at the time, Thunderstrike sold better than Thor. Even without looking at the numbers it's easy to believe! When DeFalco & Frenz left the title it fell first into the hands of Jim Starlin, who began a lengthy tale about Thor going insane. However, the art was lackluster and Starlin didn't stick around to finish the tale; the climax, a crossover event called "Blood & Thunder" was particularly loathed by Thor fans who, to this day, dub it "Thud & Blunder." Following that: Roy Thomas became the author, Thor became mentor to some uninspired characters called the God Pack, Thor donned what remains his worst costume in his entire publishing history and the art became even less appealing. Compared to the solid craftsmanship and dependability of Thunderstrike, Thor was serving some weak tea.
The decision to end Thunderstrike then seemed to be intended to help bolster Thor's flagging sales by eliminating what might have been a title competing for the same readership. However, the goal was not to interest readers in Thomas' run - he was let go as well. Instead, a young Warren Ellis came in with Mike Deodato, Jr. to tell a weird, off-beat story about Thor losing his powers due to a false Ragnarok. It's an interesting comic, but it's hard to imagine many fans who enjoyed the Lee & Kirby-style antics of Thunderstrike finding it to their liking.
Tom DeFalco probably has (or had) access to the most reliable data on what Thunderstrike had been selling. Back then, there were multiple comic book distributors and one would need access to all of their monthly estimates to have an idea of what was being sold, and even then you would only have an estimate. I don't have all of that data, so the fairest way seems to be to compare the Statement of Ownership data between the two books.
Thunderstrike published Statement of Ownership data only once, in issue #18 (March, 1995). Fortunately, Thor printed its data that same month in issue #484. Check it out:
Average number of copies of each issue during the preceding 12 months: 188,725 (Thor); 236,467 (Thunderstrike)
Single issue nearest to filing date: 59,600 (Thor); 56,300 (Thunderstrike)
Based on this they were running very close, with Thunderstrike posting a vastly superior average, but at the time Thor had taken a slight lead. DeFalco's statement seems to be authentic.
What definitely can't be right is the "more than Thor & Avengers combined" remark; the nearest data I could find for the Avengers put them at an average of 253,950 which placed them above Thunderstrike.
It's also interesting to note that a year earlier, Thor's average had been 259,383 and the nearest issue sold 225,600 - which shows how quickly numbers had fallen. No doubt it was the sudden free-fall in sales data across the line which led Marvel to think cancelling Thunderstrike would save Thor. Since DeFalco & Frenz had always maintained Thunderstrike had a set finale which they knew how to play out from the very start, it's probably for the best they ended the series while the fanbase was still engaged and without anyone taking control of the book away from them. Thunderstrike definitely had its rough patches - I try to be fair to Blackwulf but even I won't stand up for the Blackwulf issues - but in all, it was a very charming super hero book at a time when much of what was being published felt ugly and crass.